How much is a dollar in Steam?

Excuse me, could you please clarify something for me? I've been trying to figure out how much a dollar is worth in Steam credits, but I'm a bit confused. Is there a fixed conversion rate, or does it vary depending on certain factors? I'm interested in purchasing some games on Steam, but I want to make sure I have a good understanding of the exchange rate before I make any transactions. Could you possibly help me out with this?