Who says it costs even $35. Its just economics. Accessories always have higher profit margin not linked to production cost. To answer your question scientifically - Its not about what a piece of glass can do, its what it can do in addition.
1) Of course there are really cheap glasses that block UV. You don't want it to reduce transmittance of visible light and induce aberrations which is hard.
2) Adding one piece of glass in optics is not as simple as it sounds. Think, reflection dispersion, scattering etc etc. That bring coatings into picture, which means additional technology, problems (uniformity, stickiness, durability) and cost.
In the end its still just economics which results in good filters being too expensive.....