Having made cushions in my shop it is more time consuming than you would expect. Also quality foam is VERY expensive. Usually it can cost more than the fabric itself. The "marine" vinyl you get at say Joann's fabrics is not very good quality either. Those two cushions can take a good day to make properly. Figure in the labor costs to pay a skilled sewer, materials, overhead, etc. $350 is not bad depending on what materials they are using.
Iceman the thread can be VERY expensive. Some of the high end stuff is over $100 for a small spool of it.... and you need at least two of them! It kills me when I see nice new boats in the storage yard with no cover on them... There is this nice (was nice) 20' runabout a few boats down from mine. Bright yellow and white interior... I watched it fade and go nasty in hardly a year. The UV down here just eats that stuff up... It will cost thousands to reupholster the boat. Even a cheap $90 walmart cover would have saved it.
I know I am going to get chewed out for this