I am in the same boat, if someone had a $100 solution for internet (even slow) for my Amiga 500 I would totally buy it. Specially now that I ordered a ACA 500 for mine, it will be a much more useful machine.
Currently I have 2 cheap solutions, one is use a Linux computer or Raspberry Pi and access the internet via it's console shell (Lynx). This is rather more limited but it works on all Amigas even the Amiga 1000 with WB 1.3.
I wrote a tutorial on how to do this.
http://www.amiga.org/forums/showthread.php?t=67191Second is use a Raspberry Pi or Linux machine (virtual or physical) to host a PPP connection over Serial. I am still working on that since there is no clear Amiga tutorial that gives you the whole picture. But I think I am close, once I get it going I will share a tutorial with the correct settings.
Also I am sick of everyone always pointing to getting a 1200 as a solution. They are rare in North America and expensive to ship from EU. The commodore 64 has a version of Lynx and can easily (slowly) surf the internet for about $100, the Amiga 500/2000 should be able to as well.