German doesn't use a word for more with adjectives. It adds -er — however long the adjective is:

Adjective	Comparative	English
billig	billiger	cheaper
günstig	günstiger	cheaper, better value
schön	schöner	nicer, more beautiful
klein	kleiner	smaller
bequem	bequemer	more comfortable
interessant	interessanter	more interesting

Than is als: Das Hemd ist billiger als das Kleid. — The shirt is cheaper than the dress.

Never mehr schön or mehr interessant — even long adjectives simply take -er.

For prices, many Germans prefer günstig to billig: billig can also suggest cheap and nasty.