You've come to the right place: I used to sell these things.
How much should you spend. It depends upon the features that you want it to do. Remember, these things will last a long time. You will want to be happy with it for a long time. Your former microwave is a 600 watt. You will want at least an 800 watt and about 1.1 cu ft. Other options are convection,(expensive, but adds another oven to your kitchen) turntable, (a must in my book, but most MW's today have this) A good selection of programed options on the keypad ( popcorn, reheat, defrost) You will need to examine them to see what serves your family the best. If you want something neat and have money burning a hole in your pocket, then go for the over-the range type that replaces your above-stove fan. It frees up space in the kitchen. It appears to me that a basic unit is selling in the $85-200 range. Since I really don't care much for using the microwave (When I cook, I like to do it the old-fashioned, stove-top/oven way) I usually find the cheapest thing with the most features out there. If your a modern Betty Crocker that uses the crud out of your machine, then I would suggest budgeting more money for it.