I ended up getting X-COM (North America version) for $31.98, but I'm not sure if it was a glitch or what because it wouldn't work until my third try. I've never had a problem with my card on GMG before, but it failed twice in a row. When I did it using PayPal it worked fine.
What I did was pick up the free game, then clicked the banner on the right that says you get $8 off X-COM for trading in the free game, then I paid using PayPal. Unfortunately the GMG site broke (it just kept saying "We are processing your order..."), but I got a receipt from PayPal and the game showed up in my Games list, so I'm assuming it all went through. It's still weird though.
Some Maths for my own benefit:
$49.99 - 10% = $44.99
$44.99 - $8 trade-in for Age of Fear = $36.99
$36.99 - $5.01 = $31.98
I'm still not sure if this is too much for my taste, but I have high hopes for this game.
Edit: I should also mention that Age of Fear said it was $0.00 trade in, but in my cart it correctly reduced the base price by $8 even though it didn't show up as credit...