The 10% given by the economic/quantity policy is enticing, but I wouldn't waste an idea slot (quantity) just for the sake of getting that policy.
Counting that the policy itself costs about 120 mp per 10 years, I'd rather take an idea like innovative that gives me 10% on technology cost (so about 140 mp net saving in 10 years) that
also gives me other benefits (like 1 leader free-upkeep which also saves other mp).
Quantity is useless to me. Developing your provinces naturally increase both your income and your manpower (even more so counting that you have more building to squeeze even more both). Quantity might be useful in the first 50 years of the campaign, but usually during this period I give priority to other ideas. After that its usefulness starts to fade rapidly
I haven't played tall much, but I'd still rank Quantity > Innovative, probably even more so for tall play (though it's useful in wide as well). If your average autonomy is ~0% (b/c you have all states), that extra manpower and FL will give you even more troops, allowing you to take on bigger blobs / refill manpower faster / etc. Every idea in quantity (except for garrison size) is worthwhile - even in wide play where more troops => bigger targets/faster wars/more fronts. Many of the innovative ideas (prestige/inflation/WE reduction/+1 advisor) are "meh" unless you're doing some weird stacking strat. The -advisor cost idea got a boost recently with the +5 adivsors, though.
The -tech bonus is over-rated, I think. It saves 60*3 = 180 pts every ~13 years or 1.15 pts/month, with an absolute max of less than 60*3*29 techs = 5220, but effective lower than that (have to wait till ADM 5). And it costs 1200 before you even get the tech bonus. Plus, it's an admin group which means you won't be able to take it till probably your 3rd slot (after Econ for tall; assuming you're playing with group restrictions) or it has to compete with the other admin groups. Sure, quantity doesn't save you any MP, but it's other bonuses are worth it.
As for whether the policy is worth it or not, that's debatable. It saves 5 mana per development, so you have to develop 120/5=24 times in 10 years or 2.4 times a year to break even. Not sure what the development-pace is like in a tall game, but if you stack this with all the other modifiers, development should be pretty cheap, so you can develop a lot. Considering you probably also "stack" developing into bursts to take advantage of edicts/the burgers, I could easily see developing much more than 24 times in a 10 year period to make the policy worth it.