Wallet functionality's been added to Steam, allowing you to add funds to your account.
You can deposit anything between €5-100. Obviously, your currency will depend on where you live. Because that's how money works.
Check it out here.
Beware, though: once your money's in, you're not getting it back: "Funds added to the Steam Wallet are non-refundable and non-transferable," according to the now-updated T&Cs.
"Steam Wallet funds have no value outside Steam and can only be used to purchase Subscriptions and related content via Steam (including but not limited to games offered on the Steam Store)."
Still. Funky, innit.