There's little point in upgrading some spells. I have some that I use rarely if ever, why spend several gold upgrading a spell that I don't use?
Concentrate on the ones that you use the most, or that are most bang for the buck for your class.
If you are a healer, your heals and wards need to be pretty top of the line. There isn't such a significant difference between app4 and adept I that if the app4 is selling for 50sp and the adeptI is selling for 5gp that I wouldn't pick the app4.
Concentrate on your most used, most important spells first. Don't forget you will get some drops on occasion as well.
Forget master spells, if you ever see one you will be quite fortunate, but you certainly won't have a spellbook full.
And final piece of advice, figure out which spells are going to be replaced and when, and that will also guide you. If your top spell is being upgraded in 2 levels, why bother buying an expensive adept spell? If its going to be 10 levels, it might be worth it.