if you are doing the work, low end carpet will be around $300. Remember to buy the right glue. You will need around 2 gallons for a 24' boat. That's about $60 on top.
The seats will vary in pricing depending on how much rot you have in them. Marine ply is expensive. Is the foam still good? Foam is very expensive. If all you need is to have the seats recovered and everything else is usable, you will be looking around $600 - $700 in materials and $1500 -- $1700 in labor to have a pro recover the seats, sundeck, and console.
If the wood is rotten or the foam is shot, you are better off buying new rotomold plastic seats.