Okay - Green Zebra is down the street from West Town, even a little further west, so you might as well do West Town in that case.
I think it's going to be tough to find something in River North or the Loop that is like West Town in terms of it's relative approachability in food and its price. Why? The chef/owners of West Town, Drew and Susan Goss, had a restaurant in River North- Zinfandel- which was there for about 9 years before they opened West Town in about 2001. During the tenure of Zinfandel, the neighborhood, River North, had transformed from being slightly seedy to downright trendy. Thus, rents were likewise affected. The moral of the story is that most restaurants in that area cannot serve relatively straightforward food at reasonable prices and survive.
Having said that, while I love Avec and Green Zebra and Follia, I'm not sure their food or portions or price are necessarily appealing to picky, somewhat-thrifty eaters.
In fact, I'm racking my brain and can't think of anything in those areas (especially in the Loop) that meets your price point (West Town charges roughly between $10-20 an entree, including a beef tenderloin entree) and level of trendiness.
Could you just hop in a cab to West Town? It's only about 10 minutes down Chicago Ave. from the Gold Coast.