Hi,
Here in Europe for example the large hobbyshop Lindinger has the monoplane of PrometheuS listed at 2.099.99 Euro = US $2279. That is including sales tax (that is 20% in Austria for example).
Here is Sweden a Sebart reseller has PrometheuS listed at 18.690 SEK = US $2162 (we have sales tax of 25% here in Sweden on things like this).

A good guess is that a biplane will cost more to produce then a monoplane, how much more a PrometheuS biplane will cost is hard to say.

/Bo