The specs for a single run (without a repeater) of CAT5 UTP is 100 metres, and there is a margin for error.. that is at 10 or 100mbit. I think you can go a little further using coax.
However a repeater is very cheap.
Since you are in different buildings, I would go for a more high powered wireless device.. theres a few different types, but I don't know much about them.