You will have to pay for tangibles, like wood, epoxy, aluminium extrusion, sails and the sort. Plan costs are negligible compared to this and probably the least important cost. Plan for the Blade F16 for example would fit your need, those are available from http://www.formulacatamarans.com/ and I think you can get plans for his Blade 18 as well

There are some old tornado plans available there:

http://www.thebeachcats.com/modules.php?...nload&cid=6

Those require licensing payment as well once construction is finished.

I'm sure you can find more...