Yes, the initial (upfront) cost of oil can be considerable, though 6 gallons seems like an awful lot. IIR the last time we did it we used closer to 3 gallons for a bird that was ~15lbs. If you're using 6 gallons you're either frying one enormous turkey or (more likely) your pot is too big for your bird. Our CostCo sells gallon-jugs of oil for $7.
Speaking of which, a few important details
1) know how much oil you need for your sized bird and pot BEFORE you lower a cold bird into 350ºf oil. Most fires occur when the oil spills over onto the burner, the lawn, the turkey-frying-person.... Check first by submerging the turkey in water, then make sure the turkey is DRY again before deepfrying (or else it will splatter like crazy, another common mistake)
2) You can (and should!) re-use the oil. Post frying and cool-down, strain it with a fine-mesh strainer and/or several layers of cheesecloth. Filtered oil is good for several months.
3) Don't be a frying-idiot-statistic. Do it outside, with nothing flammable within a 6'+ radius. Wear heat-resistant gauntlet gloves, cover exposed skin and wear an apron. Turn the gas OFF before lifting the turkey out (so you don't get oil dripping onto a flame).