Show webformblock at products with price 0

Apart people selling product for free, for example free downloadable products or catalog and samples, in other cases, I think could be useful to show a webform for products without price, for example on my site www.ebagno.it selling bathrooms, often there are products without pice, because it come out froma phone consulting with lcient and pricecan't go online published, because it is never a standard price for some products.
So how we could show a webform to ask quotations, but only for some products and not for all products?