@Carol91321 typically the most affordable long underwear will be made from synthetic materials (nylon, polyester) and often our REI Co-op brand will be a great price point. Since warmth matters, you want to consider at least midweight. Synthetic materials wick well and dry quickly, although they can retain odor over time. Natural fibers (wool, silk) also wick well and dry quickly and do better with odor control; wool is also generally regarded as the 'warmest' material for base layers, although it is often also the most expensive. We've got a helpful Expert Advice article about selecting base layers that might help further; hope this helps!
@Carol91321 Depends on what you consider to be " a fortune" and where and when the backpacking is to take place but I like Merino wool for that purpose because it performs well if wet and is naturally odor resistant.
Smartwool 250 weight is probably among the "best" for general use...some family member of both sexes used this in the Sierras last summer mostly as night wear...it often drops below freezing there at night and there was sill quite a bit of snow laying around.
but REI brand merino wool 200 is a good value. I have the half zip top and would buy the bottoms but some ancient medium weight REI synthetics are still working for me so I haven't.