With a sealed 12" sub, you're not going to get a lot of output down low. Here is a comparison of a 6cu ft ported enclosure and a 2cu ft sealed, both with 500w.
There's also the 4cu ft ported flat pack
This is a slot port
The SVS SB12 is a sealed 12" sub with 400w. If you go with a Sealed 12" Dayton Ultimax with 500w then I think you should get similar performance. It all depends if you want to take the time to build something or buy something premade like the SVS.
That is about $434 without glue, paint, binding posts, and polyfill. In the end you're probably looking at $500-$550 depending how fancy you want to get finishing the enclosure.
If you want to bump it up a bit then there is this kit
and last one...