If you interact with the right people (by writing meaningful comments, of course), they support you either by upvoting your comments, or giving you various credits.
The low amount of Resource Credits (RC) is a problem only in the beginning.
I give you some !LUV and !PIZZA.
As for encouraging spam by tweaking the costs, you may be right.
Nowadays my upvote is starting to worth something (recently reached $0.01), and the spammers already started to appear under some of my posts. If the cost of commenting would be less, then there would probably be more spammers.