I agree with Blue Topaz... its all about research and trying out different brands of hair (most suppliers will send you some samples) ... wash the hair a few times, straighten, curl and comb the sample hair and see how it all reacts/lasts or as blue topaz suggested.. wear the hair yourself or apply it too friends and family and get there opinons on it!
Hair which is too cheap, is more than likely going to be very poor quality and im afraid it tends to be the saying: 'the more you pay, the better quality you will get'...
if its really cheap hair, then theres usually a good reason why (it will more than likely tangle/matt/dry out etc)...
Most people/suppliers claim to be selling '100% virgin' or '100% Remi' but the truth is, its not alway what you will recieve and after a few washes, the condition of the extension hair will dramatically change from ok to worst..
I have done so much research over the last few years on different brands of hair extensions and i have come across some absolute great hair suppliers and some very awful, so do your research and stick with the best hair you can find... at the end of the day, SOME of your clients might be a tad put off if you use expensive hair (and obviously charging abit more for the fact that you use expensive hair) as not everyone wants to spend £100's, but the ones that do pay you the money for using more expensive hair, they will thank you in the long run as they will have hair extensions that last the 3 months and even more, plus it will save you so much time, money and hassle.. dont be fooled into using really cheap extensions hair otherwise you'll have nothing but trouble in the long run!
Hope that helps in some way. Good luck
Laura xx