It doesn't cost the upvoter money. In fact the upvoter earns rewards (STEEM Power) by upvoting.

How it works (VERY simply)... New STEEM is minted every day. Some of that newly minted STEEM are allocated the people that post, comment and vote on Steemit.

How much of the allocation is awarded to a given post will depend on the weight (STEEM Power) of the people voting on it.

Successful bloggers and curators are being awarding new coins, in the same way that Bitcoin issues new coins to miners that successfully discover blocks.