No, you do not have to pay for it later. This is one if the things I watch for when searching apps. If I find one I "have to have" I throw it on my Wish List in AppShopper and wait for a reduction in price.
What really blows is paying $4.99 for an app only to have it drop to $.99 or Free the next day.
The app will be listed in your iTunes account as having been purchased already. You can then redownload it at any time. The other thing I do is if I have downloaded a number of apps during the week or so, I will just sync my iPad so the "purchases" transfer to the PC, saving it forever in your iTunes library. But, not everyone uses iTunes.