I wouldn't buy cheap fresnels before I can see their lightbeam. I compared a cheap Eurolite fresnel to a Ianiro fresnel side by side and the results were very enlightening. They both used the same 1K tungsten lamps, but the output of the ianiro seemed to be at least 1/4 more. The ianiro has a perfectly even output, and if you focus the beam it narrows and stays a perfect circle. The eurolite is not as even in flood, and if you focus the beam it becomes some undefinable blob that looks more like a "8" than a perfect circle.
The Eurolite is a super cheap fresnel, under 100 Euro, while the ianiro is over 700 Euro - but you can definitely see the difference.
There may be good and cheap fresnels, but I'd rather have a look at them before buying.