Check your local craigslist, check your local pawn shops. Make sure you know what you want, and how to haggle. Everyone asks for too much, but there should be a flood of used laptops on the market since it's in between semesters.
Otherwise, your best bet would be a chromebook running chrubuntu(not crouton), however, there are some caveats:
1. There won't be a backlit keyboard
2. You won't be able to get a great processor, but it'll be able to handle ubuntu just fine. You may want to consider a slimmer desktop manager than unity.
3. You'll be limited on browser tabs before your memory runs too low and things slow to a crawl. Extensions like The Great Suspender for Chrome[1] allow you to work around this somewhat. It requires some manual tab management, but the alternative is maintaining a bookmarks folder so you can close the tabs and come back to them later.
4. I was able to get an IDE(PyCharm) running, and it worked pretty well, except being slow in code completion.
5. Inkscape runs, though not well. I'd suggest you running the app by itself if you need to use it.
[1] https://chrome.google.com/webstore/detail/the-great-suspende...
(edited for formatting)
