Not to miss restaurants for out of town guests
Summer Shack is always a hit - you have to choose carefully to stay within budget, but it can be done and it's never been anything but extremely popular with out of town guests - Alewife more often, but Back Bay is OK as well. Kingfish Hall is fine as well and the food is generally better, but it will be more expensive.
on a nice day, Dantes, on the patio. Where are they from and what don't they have back home? We have some very good Portuguese as well as, of course, lobsters, fried clams, etc. but if they don't have good French at home, try La Voile or maybe splurge on a Chef's Whim night at Craigie Street.
I (kinda) hate to say it, but out-of-towners should be taken to Legal for some clam chowder and fish. Of the locations I've tried, I like the one by the Aquarium best - really nice decor, good service and fresh fish. Another fish option is No Name Restaurant by Fish Pier in the Fort Point area. Both will definitely hit your price-point, though NNR will likely be the better deal for you. It's definitely nowhere near as nice as LSF, though.
The Parrish Cafe is also nice for an informal lunch/brunch, especially since they have lots of sandwiches made by other city chefs. Kind of like going to a few restaurants in one place, but not really. Another option that I like to bring friends and family to, but have been shy to admit on this board, is Kingfish Hall in Faneuil Hall. The calamari app is really wonderful, as are the wasabi-crusted tuna and the snapper. They have a little asian kick to them, which is a nice change from the ordinary. Hope this helps!