As your post shows, the base clock isn't the only factor in performance. Actually, for almost a decade the fastest stock clocks have been above 1000 MHz, Radeon HD 2600 and 2900 from 2007, GeForce GT 430 from 2010 being the first four digit ones I could find.
And I repeat, the alpha is still a work in progress. So far the only thing for sure is that the best card in the market will run AH3 very well. Most likely the second best would do equally well. Your question could be translated to: "What would be the cheapest card to run AH3 at my desired level of quality?" That question will remain unanswered until the release of the final product.
A known fact is that the new game will be more graphics intensive as the alpha shows, so the better card might be a better choice. Then again, until the release there will be changes in the video card market, too, maybe changing the pricing towards your budget.
If you can, wait. If you can't wait, get the best you can afford.