grepwords.com is another one. It's by by Russ Virante, who I have a lot of time for and although he is now at Moz, is truly a brilliant technical mind in our industry.
All the ones mentioned above, as well as Grepwords take their data from scraping Adwords... That's now been severely restricted and as such the data view is much harder to liberate and in time the prices will go out of sync.
My view is to work with one DB and treat it as a snapshot in time. Prices may change on an individual keyword basis but overall the prices will remain relative to each other.
A final choice, if you're prepared to pay and get all the data in one go is Xedant / Pastukhov - It's not "low cost" but is is pretty comprehensive.