I assume you mean under $20 total including tax/tip, so you're looking for main courses around $15 or less.
You can go to Adobo Grill (1610 N. Wells, right next door to Second City) for under $20 if you stick with the enchiladas entrees. www.adobogrill.com
Otherwise, maybe Nookies (1746 N. Wells)? http://chicago.menupages.com/restaura...
Trattoria Roma on Wells just south of North has several entrees under $20.
Angelo's Taverna a few blocks west of Second City near Sedgwick and North has Spanish, Italian and Greek small plates generally around $10 per plate. No website I am aware of but the address is 1612 N. Sedgwick.
There are several bars across Wells fro Second City that serve decent pub grub. Wells on Wells and Coccoran's Grill come to mind.
Can't find Corcoran's web page, if they have one.
Further south down wells are several other options, but with the snow we are expecting I hesitate to mention them.