So you want a company to make for you custom interference filters, CHEAP? First look at prices of standard, off-the-shelf components here:
http://www.edmundoptics.com/optics/o...e-filters/3196
(and elsewhere in their online catalog). 25mm diameter: $99, 50mm diameter: $245.
Don't know if that fits your definition of "suprisingly cheap". For 50mm diameter, definitely more expensive than the average photo filter, even from B+W.