。
Non-members pay an additional $50.
Literal
Non-member [topic-は] 50 dollars additional [target-に-become-となる] [polite-ます].
~となる is the formal version of ~になる ('become') with と instead of に. Used in written/formal contexts, particularly pricing, announcements, and bureaucratic language. 増し ('addition, extra') is a noun/suffix from 増す ('to increase'). ドル is the katakana for dollars. Typical registration-fee notice.