A high bay light is normally an industrial or commercial installation that is hanging from chains or a hook. to find some that are true high bay (proper reflectors, etc) that are designed for recessed installation into a ceiling is probably asking too much.
T5 ballast generate lots of heat, and however you install them, the ballast and top of the fixture needs to be exposed for cooling, and certainly not covered with insulation.
While the T5 bulbs are indeed expensive, they do put out the light, and unless you are going to be running the lights all day every day, you may be years before you replace the bulbs. Cost of the bulbs is probably the least of your worries.
Charles