You can get much better prices if you buy them in quantity, or shop on
eBay (carefully).

But the whole idea of repairing ballasts is fraught with hazards-- I
don't know the laws in your country, but here if one of your ballasts
ever burns up and causes any damage, you're probably personally liable.
If you're doing this at your work, I'm pretty sure your company's
insurer would frown on this. There's just too much risk (millions of
dollars) for the savings (very small, if at all). Even if you value
your time at $5 an hour, you're losing money on each one.