Find your parts in used condition on ebay.
Then filter on completed auctions,
In green you will see what the part actually sold for.
But... be careful what you wish for.
A R9-290X is in the same performance tier on Tom's hierarchy chart.
The difference between 3gb and 4gb vram is not meaningful:
http://www.pugetsystems.com/labs/articles/Video-Card-Performance-2GB-vs-4GB-Memory-154/
Meaning that you can't tell any difference in performance without a synthetic benchmark.
The R9 cards seem to be noisier, more power hyngry and less satisfying.
On newegg, your card has a 2% unfavorable rating, the most popular R9-290X is 24% unfavorable for whatever reason.
And... they are more prone to failure
http://www.pugetsystems.com/labs/articles/Video-Card-Failure-Rates-by-Generation-563/