Hard to beat Walmart. I get the Federal 36gr plated, 550 round bulk carton (red box) for $19.99. Its not the cheapest they have but it is some of the cleanest burning 22 ammo I have run across. My guns are easily twice as dirty with most others.
Want to go cheaper? They had some Winchester either 500 or 550rd boxes for $16.97 and the Federal Auto Match was about $12 for a 335rd box. Hard to get anything worth shooting for less than 3.5 to 4 cents per round. The Federal works out to 3.6 per round before any sales tax
I ordered a bucket of the Federals from Academy last spring. It contained 1100 rounds and was $29.99 with free shipping which comes to 2.7 cents per round. Keep your eyes open for deals like that and sign up for email sales announcements with multiple online dealers like Blazermark does.
If you don't mind buying in bulk, TargetSportsUSA.com has the best delivered price of anybody I've found, i.e. shipping is free on bulk ammo. They also have a Prime program a la Amazon.com that gives you free shipping on non-bulk ammo and discounts on everything (including bulk ammo), as well as some free ammo once a year based on your purchase history.