Typically you would only be able to sell your collections to people who regularly read and enjoy your comics. If you are publishing a paid collection without a free collection first, you're unlikely to sell any if your web comic is family-friendly - not sure how well the adult-only section of this site does - I have the blocks ticked to never see any of it.
Else try Smashwords. They do have people there that will read books / comics etc. You can also browse their catalogues and get an idea of the price. They also have regular sales where you can discount your books up to 100% off.