Best API for keyword tool

Hi guys I was wondering if anyone here had any experience building a keyword research tool?

I am building a keyword tool, not so much one to research keywords (although this will be part of it) but more to manage the process of ranking for keywords (i.e. managing the tasks and monitoring the results etc).

I want the users to be able to add their own keywords to the tool but I also want functionality similar to the Google keyword tool where it suggests broader keywords from source keywords and shows number of searches etc.

From my research so far I've found the following - can anyone help in terms of validating these assumptions or otherwise?

Google Adwords API - Kind of seems a bit more complex than a typical API but I've signed up (which apparently takes 1-2 weeks) and I also read that it costs money. This kind of surprised me as I want to make the basic version of my keyword tool free so I don't want to pay for keyword searches.

Wordstream - I've done a quick test with their API seems a bit limited, I've contacted them to ask if it can return exact match search numbers and can be tailored for different locations. And I've also enquired about the price. Again I don't really want to pay for this functionality but if this tool is substantially better than the Google keyword tool then I'll consider having it as a paid extra.

Wordtracker - Pretty much as above except I have just contacted them - I haven't tested the API.

I guess my frustration is that I see a lot of other keyword research tools out there that don't charge monthly fees but do offer this functionality so I'm wondering how they are doing it.

Thanks

Dan