It may be a good idea to distinguish if your T5's are NO (normal output), or HO (high output). A t5HO bulb runs at a higher wattage than a t5NO bulb. If memory serves me correct, a 4ft. t5NO uses about 28 watts and emits approx. 2900 lumens. A 4ft. t5HO bulb uses about 54 watts and emits approx. 5000 lumens.

The t5NO ballast used for 4 ft. bulbs would not be able to drive a t5HO bulb properly. Can your t5 bulbs be used in a less expensive fixture? Yes they can, if the fixture is designed to use that specific bulb.

Less expensive does not have to mean "cheap". Cheap fixtures will have low quality ballasts and poor (not very efficient/effective) reflectors. Often times, the differences between a quality fixture and an inferior fixture will mainly involve ballasts and reflectors.

o2tiller has a good point, is there a model number on:

1) Fixture
2) Ballast(s)
3) Bulbs

I think we can work a solution from there.