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Just for comparison...
2014 YZ450F msrp = $8599.00
2014 YFZ 450R msrp = $8799.00
So purchase cost is very close, though bikes seems to depreciate faster than quads. As for maintaining costs, I'd honestly say the quads are probably less..unless you crash a lot and break things. You'd think tires would be be a big cost factor, but they are about half the cost of dirtbike tires each, and they last twice as long..so it's a wash. I can only speak for Yamaha's as far as quads, but after the initial purchase price, the cost of ownership for me has been very very low.
It's best to do what you are doing and rent one a few more times to make sure you're going to like it before making the investment. If you learn to ride it properly, they are a lot of fun
Yamaha