Yes, it is. It is the comparative form of the adjective costly (i.e. more costly).
costlier
costlier
costlier, costliest
costlier
diamond
Platinum is costlier although gold is a symbol of wealth and money and many people believe that gold is the most costliest metal.
Flounder.
Rolex watches are costly.
costlier, costliest
there running out
Sometimes
Yes