There are quite a few specialists in the field who can get RETS data into a DB. There's also open source projects like phrets, vieleRETS and librets that can handle a lot of the heavy lifting for you.
I've used all of them at various times. I like phrets personally, since it's more of a wrapper for RETS in PHP, which makes it really easy to deploy on other servers. If you are working only on your own server, librets might be a better option, since you can compile for a variety of languages (ruby, java, C, etc). It may be a bit more powerful than phrets. VieleRETS is the most "product"-like but that's also a downfall in my opinion, since it's less flexible than the others.
Anyway-- depending on the DB scheme you need to import into, the provider you hire will likely have to do some mapping to meet your schema, which may make it almost as cost/time effective to do it yourself using one of the libraries I've mentioned.
Good luck!