Sounds awfully high to me. Not the dollar amount, which is just a few hours work in most shops, but the fact that you had to have work done several times in 18 months. Are things actually breaking, or are you paying for stuff you could easily do yourself, like lubrication and cable adjustment? I haven't taken my bikes to the shop in probably 25 years, except for wheelbuilding, so I could be out of touch on the cost. But I've probably ridden 6000 miles in the last three years (I'm just a recreational rider now, and don't keep close track), and I don't remember doing anything to the bikes beyond lube, tires and minor adjustments.
As for the rear derailleur, I've probably ridden close to 100,000 miles in the last 40 years, and I think I've only worn out one, an old Huret. I've ripped them off against rocks, bent them by getting sticks caught in the chain, damaged them in falls, but I can't remember wearing any out. That doesn't mean you don't need one, but I'd ask what was wrong.