Can I upgrade from 10.5 to 10.7 without installing 10.6 first?
The cheapest way to upgrade seems to be to buy and install Snow Leopard ($29 here) and then download and install Lion from the App Store ($29.99 [App Store link]). This totals to just under $60.

The other alternative is buying Lion on a USB sick. It's $69 here. According to the page, Apple seems to want you to buy Snow Leopard before using the USB installer (although you should be fine skipping SL technically), making this even more expensive.

If you really want to have a physical USB disk with Lion, you can make your own either before or after you install using instructions you can easily find here (on Ask Different) or elsewhere on the Internet.