Just how low will the most basic, and most restrictive, economy flights get? The answer depends on just how much you’re willing to give up to fly on the cheap.

Just this week, American Airlines announced plans to offer so-called basic economy fare on select routes from airports in Dallas, Miami, Philadelphia and Charlotte, N.C. United made a similar move on some of its Minneapolis flights to destinations such as New York, Chicago and Los Angeles.

“The launch of our Basic Economy product is transformational—offering customers seeking the most budget-conscious fares United’s comfortable and reliable travel experience across our unmatched network of destinations,” said Scott Kirby, president of United Airlines, in a statement. “Basic Economy lets you go where you want to go at our lowest available fare while enjoying United’s Economy cabin and the exceptional inflight service that comes with it.”

Those reduced fares follow other major airlines such as Delta, which began its own basic fare class last year. And that’s already on the heels of airlines that have built a reputation on offering the cheapest, no-frills flights such as Frontier and Spirit.

So just how much money are we talking here? In general, about $15 cheaper than standard tickets, according to Forbes. Here’s the catch: In most cases, you can’t select your seat, you can’t store anything in the overhead bin, and you’ll board last.

This means that if you’re flying with family or friends, chances are that you won’t sit together. And, if you can’t tuck your tiny, carry-on item under your seat, you’re out of luck. You’ll need to check that at a steep price.

What’s more, these cheap airfares will be hard to come by. Airlines are offering a very limited number of basic economy fare seats, so don’t expect to find them on every flight on the discounted routes.

Bargain hunters might rejoice, but you may get more than what you bargained for.