Hey, welcome to the board. Please let us know a little about you, your fishing style and where you're from. In general (and as a 21SE owner) I agree with you - the 1801 will be too small. As for how much - New 21SE ~40k with everything including trailer, taxes, tags, electronics, etc. You could probably get a good used one for much less if you looked around a little.
Thanks Timbo! I'll be fly fishing/light tackle along the bays and rivers along the Jersey shore, but would like to venture out into the ocean when conditions permit. I'm leaning toward the 21SE, but the 1801 is more in my price range. I probably would never have more than three people on board.
By all means get the 21 SE if you can. I've had both the 1801 and now have a 21 SE. The 18 is a great boat but the 21 SE with trim tabs is far better in the ocean and you can still go to all the same shallow places the 18 will go.