Unfortunately, due to etherium eating up all the former bitcoin/litecoin etc mining 7950, 7970, 280, 280x flood that was on the market (They were ~120 USD or less after the first wave), the best performance per dollar is currently ~200 USD R9 290s on ebay.
Everything below 200 USD is basically a gaming video card value dead zone currently.
An overclocked 2500k (or better) also pairs quite nicely with a R9 290, without the R9 290 being bottle-necked massively (Still pretty bottle-necked though), and the recommendation for budget gaming rig has a clear lower limit for value purposes.
http://www.ebay.com/sch/i.html?_odk...0+-"R9+280x"+-"R9-280"+-"R7+240"&_sacat=27386