A 7.5 gallon kettle is (barely) big enough for 5 gallon batches, so you really just need a propane burner ($30), mash tun ($30-50, depending on how you build it) and basic immersion chiller ($50). Considering you save $10+ on extract each time you brew, the basic equipment isn't hard to justify IMHO.
As Denny pointed out, you could also do partial boils and put off buying the chiller and burner.