Hi Jacki
It's a hard question to answer, as it depends so much on where you eat, and also what drinks you have with the meal.
US meals are very large, so if you eat a "normal" US breakfast, I doubt that you will need much at lunchtime.
For the majority of cities $35/40 (around £20/25) will get you a nice meal and a drink, including tip. You can spend a lot more if you want, for example you can spend any amount you like on a bottle of wine, just as you can in a restraurant at home, or on things like seafood if you're in a costal location (although this will still be cheaper than in the UK)
You can also spend a lot less if you want, with any number of fast food establishments or "sandwich shops" around everywhere. Also because of the size of meals, I very much doubt you would want a starter, main course and sweet, which helps to keep the cost down.
Hope that all helps.
Stuart