You might want to take a look at Tuft & Needle. They make a great all foam mattress for $400 and make a great guest room bed. Check out the "Five" on amazon.

The Sealy Amici firm or the Sealy Wallach plush...both at Sears....the best you'll find for that much money.

Sean's advice is good, Tuft and Needle is good for the price

You can't buy Tuft & Needle unless it's sight unseen. If you want to actually test it out, I recommend private labels and off brands at your local retailers. Southerland is great if you have a retailer that carries them. But you'll get better bang for you buck with the off brands at that price.

Tuft and Needle is probably your best bet in this price range - they are coming out with an updated model in Nov/Dec 2014 however, which they say is going to be a big improvement over the current model - so if you can wait, I would.

Comfortable is a relative term. What might be comfortable for you might feel horrible to someone else. You always want a guest room mattress to be comfortable enough for you to spend a few nights on if need be, but not so comfortable that guests overstay their welcome! Haha. Almost every retailer has at least 4 or 5 average quality mattress sets for about $500. If your idea of comfortable is on the firmer side, you'll have better luck with that price range. Softer beds have more foam, padding, and cushioning which typically brings the price up higher than firmer mattresses. You didn't really specify the size you were looking for, so I think we're all assuming queen (most common bed size). You can find either lower end name brands, or as Andrew suggested, you can find some great quality non-major name brands like KingKoil, Englander, Restonic, and several others. You could also get a nicer mattress if you're willing to sacrifice a little bit of size if you wanted to possibly get a full size mattress set instead of the assumed queen. Just a thought. I hope this helps you, and have a great day!