The cheapest method would probably be to do something like this :
go to this site -> https://localbitcoins.net/
it's all people who are selling bit coins individually, some take PayPal some take cash, some take credit cards.
After you've acquired the bitcoin send it through block trades and convert to steam.

That's probably your best bet unless you have a coin base account and could do it through that ( Coin base is what I found have the least fees while purchasing coins

Here's my coin base link, if you deposit $100 you get $10 extra for free (so that might also cover your fee)
https://www.coinbase.com/join/5a3dc0f0f03db50124095ab8

Good luck and if you find something else let me know!