I don't have a clue!! But it might help if you describe what kind/type of boat. Size is obviously a huge factor in cost and by just saying it is an 18' boat doesn't give all the details. Maybe you could post a picture and someon can help. Good luck!!
To re gelcoat a boat is a royal pita! Almost everyone repaints with awlgrip, awlcraft, sterling, imron.. Much easier to respray and lasts longer. Boats are only gelcoated because the gelcoat goes in the mold before the fiberglass and when they pop the boat out of the mold the gelcoat is already smooth and shiny. Minus some wax and pva..
Your best bet would be to try and do it yourself. You could get away with using a quart perhaps and there are plenty of good kits on the market for under $100. Add in the infamous free time and labor for yourself and you can get off pretty cheap. I personally would not go with gelcoat for the PITA reasons. Plus, I've never seen anyone really utilize it on an aluminum boat. Good luck.