If you're not doing corals (and I suspect you're not), then the cost isn't nearly that high. The major expense is the live rock ($5-10/lb, depending on where you are... you'd want about 8-12lbs); you don't need upgraded lighting or anything for just the fish. You actually can do plants in SW, but it's not like keeping FW plants... and you'd need that upgraded lighting to do them. And really, a 10g setup is going to cost just the same as a 2.5 or 5g setup, so you might as well get the extra room.
Just want to throw out there, a neon goby should NOT be kept in a 2.5g, they are very active little fish... you'd want a clown or some type of perching goby for such a little tank.