i have only seen one review where someone actually benchmarked the xfire set-up and it was actually pretty encouraging. for games with a xfire profile, there was almost 80-90% gain from the crossfire.
the 750ti is a card with a specific purpose, which is low power upgrades. if you are running a 300-350w power supply like in a prebuilt system then it is a good option. otherwise there are better cards for the money. for instance, the r7-370 will beat it no problem for the same money and often you can find a gtx 950 for little more which will beat both cards hands down.
sadly some people just got some kind of a tech boner for the 750ti and it gets thrown around as the greatest card ever made rather than the niche product it is. looking around a bit, this is the best card i see for the below $150 budget you have. you can probably find better with an open box or refurbished unit but a quick look shows this as a good option.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814125809&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID= it's a gtx 950 for $130 after rebate. it sips power so no problem on a 600w power supply (even a cheap one) and will beat both a 750ti and the crossfire option with the a10.
considering you have the budget for it, i'd go for this option of the others