2003 Kia Sedona Won't start

Electrical problem
2003 Kia Sedona 6 cyl Automatic 81,000 miles

I have been having problems for about a week with my car starting. Sometimes it turns over immediately on the first try, other times it seems to crank a few too many times. My neighbors son is a mechanic and told me it sounded like I needed a new battery. I took it to an auto supply store and they installed a new battery. We started the van to check it before I left and it started immediately, but the engine was revving at around 4000 rpm. I had to "kick" it down by depressing the throttle; then it was fine. I went inside the auto parts store to pay my bill. When I went outside to start my car it started right up but would not stay running unless I kept my foot on the gas. This happened repeatedly and the car would not stay running so I had to call a tow truck to bring the car to the dealer for repair. When he finally arrived 2 hrs later I got in the car and attempted to start it and it started and stayed running this time. I opted to have it towed to the dealer as I had already paid for the tow and I didn't want to get stuck again. I am a carpool Mom. Now the dealer tells me it is the alternator and the repair will cost almost $500. I'm not a mechanic, but logically this doesn't make sense to me. I'm worried that it will not fix the problem. When I was stuck there was a man at the store who glanced at my engine and stated that my throttle cable was loose and might be contributing to the problem. Any thoughts? That's alot of money to spend if it is not the real problem. I don't know these mechanics very well as the shop I used to take my vehicle to ( a Kia dealer) closed and this new dealer just opened.
Thank you.