It's not uncommon to find mixed tyres front and rear, they're going to wear at different rates so plenty people end up just shopping for the best bang for the buck tyres for either the front or rear axle, not expecting it to make too much difference. I know before I joined this forum I'd not really though about it being a problem.
On a slightly related note, I hear from people at work all the time "oh they're only tyres, I'll just get the cheapest ones around, it doesn't really matter". Those four bits of rubber are the only thing connecting you to the road though. Maybe it depends what you're driving, but I think it's worth paying a little more for some that will actually keep you there rather than in the nearest tree!