The big question to ask them is what the R value of the insulated siding is. For scale, a 2x4 wall with fiberglass insulation is about R-13. A 2x6 wall with fiberglass insulation is about R-19. Most building codes call for attic insulation to be at least R-30.
So if their insulation is R-2, then you aren't getting much insulation. It's better than nothing, but not much. If you don't have modern, insulated double-pane windows, spend your money there first.
A few things on the "cheaper" front that really help:
Put an insulating insert like these on any electrical outlets inside your house that are located on exterior walls. They go right behind the faceplate. There's similar products available for light switches, if you happen to have any in an exterior wall. The reason this is so helpful is the electrical box takes up insulation space in the wall, and frequently builders don't bother insulating sufficiently behind the box since it can't be seen.
Also, go around the house and fill any holes between conditioned and non-conditioned space - for example, if there's a hole from an unheated basement into the house, stuff some fiberglass in the hole, or use a little spray foam insulation. (If you've never used spray foam, only use a very little amount. The stuff massively expands)
Go around and check/repair the caulking around all the doors, windows and any holes around the outside of the house.
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