If it is an all metal radiator (rare), then a knock-off wouldn't be bad since you can braze them to repair. However, if it is a plastic end tank POS so common on newer cars, my feeling is that you should pay for quality. Those things suck on a good day, buying one of questionable quality is inviting problems.
If you don't have to worry about rules, check out a circle track racing catalog (
www.ColemanRacing.com or
www.lefthanderchassis.com for example) and compare their off-the-shelf options to what you run. I found that my Focus radiator is remarkably similar to a universal GM style 19" x 27.5" unit. I would need a hose adapter or two and a couple hours to work on the mounts, but at less than $200 it is about the same cost as my OE and I would presume since it cools built short track V8s well, more than capable of doing the job for my little old Zetec. Perhaps you can find one for your car.