Do you budget for airfare and hotel but treat the rental car as an afterthought? It is time to change your ways, because that Dodge or Ford you use to shuttle around could be the most expensive part of your summer vacation.

A dearth of rental cars and an increase in demand means prices are soaring. "It's Econ 101," said Sara Rathner, travel expert at NerdWallet, a personal finance website and app. "I am going to Yellowstone in September, and the most complicated part of the planning is finding a rental car within our budget," she said.

On one weekend in February, 18 of 20 airports in Florida were sold out of rental cars, rates in Phoenix approached $200 per day and rental cars on some Hawaiian islands went for $400 to $500 per day, according to Jonathan Weinberg, CEO and founder of car-rental comparison website AutoSlash.

Blame the stunning rates on a massive sell-off of cars.

After the pandemic hit, demand for rental cars dropped 90% in a matter of weeks, Weinberg said. Since cars were off the road, rental lots filled up. Companies had to find new places to park them, from grassy fields to stadium parking ramps. Meanwhile, lease and insurance payments carried on. Without knowing when demand would return, companies did what they had to do: They unloaded the cars — the most expensive, if most vital — part of their operations.

"Avis Budget [Group] sold off 31 percent of its fleet, Hertz probably sold off more," Weinberg said.

With vaccinations rolling out, people are acting on their pent-up desire for vacation — or "vacci-cation" — by heading to popular destinations such as Las Vegas, Florida and Hawaii. AutoSlash saw a huge uptick in demand for Presidents' Day weekend. Weinberg expected it to slow down after that, but it just kept surging.

Watch out for summer, he warns. People are sitting on huge amounts of unused vacation days, the weather is warmer and more of us will be vaccinated. "Demand will be everywhere all at once," Weinberg said.

Here's how to make sure your summer vacation dollars aren't all drained before you leave the rental car counter.

Book early: Those $400-plus-per-day rates in Hawaii went to the people who failed to book ahead. Last-minute planners at vacation spots across the country were stunned to find near-empty lots, and they paid for their laid-back approach.

Extend your search: Faced with reduced inventory, some car rental companies are dismissing searches of just a few days, holding out for more lucrative longer rental periods, Weinberg said. When AutoSlash staff sees a "no cars available" notice while searching for a weekend, they expand their search by a few days. Sometimes cars are indeed available — and some companies will credit drivers for unused days if they return a car ahead of schedule.

Let taxis or ride-shares help: It can be nice to drive away from the airport in a rental car, but that perk often comes with a higher price tag, courtesy of airport taxes. Renting from a location in town is almost always less expensive. Explore what it might cost to taxi to a rental office away from the airport. "Factor in convenience, time and effort, because your vacation time is limited," Rathner said.

During trips to cities, Rathner sometimes strategically pools far-flung outings so she can limit the number of days she rents a car, even if that requires an occasional Uber ride. "You might be spending $40 a night or more for parking the car at the hotel and you might not even move the car for the next three or four days. It is something to think about."

Try Costco or Sam's Club: These membership-only big-box retailers have relationships with car rental companies and often land deals for members.

Check your credit cards: Rathner has booked cars using her Chase Sapphire rewards card (which also includes primary rental car insurance). Its online travel portal allows card holders to pay for a rental using points. Even when paying with cash, they may be able to land lower rates. The same is true of other Visa Infinite and Signature cards, which partner with rental car companies to offer discounts and may include secondary insurance.

Use AutoSlash: This site, which Weinberg started in 2010, asks you to input your various memberships, such as Costco or AAA, and then scours offers and coupons to send you the best booking options by e-mail. The site also tracks prices and notifies you if it finds a better deal. Weinberg recommends opting for the pay-later option, even though it may cost more. Unlike with airlines, you tend to lose the bulk of the money paid if you wind up needing to change your reservation.

Join the club: Sign up for rental car loyalty programs. They are free and membership allows you to bypass the line and head straight to the parking lot to choose a vehicle. "Lines will be long this summer," Weinberg warned.

A case study: I considered my upcoming seven-night summer trip to Santa Monica, Calif., to be a virtual cost-free week at the beach. As part of a waylaid vacation originally planned for spring break 2020, the hotel and flights have been fully paid. The only thing missing was a rental car, but when I hopped online to begin my search, I was shocked. Hertz would rent me a midsize SUV at Los Angeles International for $1,094. That's for a week, not a month.

AutoSlash's deals for an intermediate SUV at the same airport ranged from $505 with Midway Car Rental to $869 with National and $1,057 with Hertz.

Costco offered rates for an intermediate SUV ranging from $599 with Alamo to $748 with Avis.

But I could take a taxi to the hotel and rent from an Enterprise office about a mile away. According to taxifarefinder.com, the one-way trip from the airport to the hotel would run between $43 and $72, depending on traffic. That hurts, but the cost for a midsize SUV for five days, picking it up the day after arrival and returning it the day before departure, is an easier $456.