I agree with those saying there shouldn't be a cost in mp for buildings. As someone said in another thread, it makes no sense to be less efficient in maritim technology because of building ports. You can apply this reasonning to all buildings, maybe even the special ones.
What makes sense is losing mp points because you would recruit an admirar, which would thus not be a thinker in a naval school but would nevertheless, given battles, improve the naval tradition of your country. Same thing with generals. Ideas could also be seen as practical thinking, but I'm not sure how.
I never loved much the mp system. I can try to work with it, but it linked with building constructions makes it present everywhere.
Why buildings are bad, in a nutshell:
1. If you're in a low tech group, you generally cannot afford them, at all, because of the MP cost. Lower tech group = anything Muslim tech or lower. You're barely on par with tech and you are scrounging to fill critical idea groups. Add in stability costs, peace treaties and military leaders, and buildings are the bottom of the barrel. So basically 60% of the countries in the world are best served by spending monarch points on other things.
2. If you're poor, you cannot afford the cash cost to build them. The base ones are too expensive (tiers 1-2) and they also come too late (especially tier 2). So another 10% of the countries in the world cannot build them. If you're poor and in a lower tech group, don't ever think about buildings. You need troops and provinces,
NOW!
3. If you're rich and big, you can expand mindlessly and it's better that spamming buildings. You can handle the rebels which you usually have from conquering new provinces and the benefits provided by these new provinces are stronger than what buildings offer. Conquering is also more fun
4. If you're rich and small - this is one of the few scenarios where buildings can help. Maybe there's no room to grow due to powerful neighbors. Maybe you don't have a lot of time to fight more wars due to time spent focusing on your trade fleets or on your colonies. Maybe you want to role play. But how many countries are in this situation? 10%? Perhaps less?
IMO:
Make lower level buildings cost fewer ducats. Make the ducat cost increase at a higher rate. Also don't have a flat fee of 10MP per building, scale it per level.
Tier 1: 10 ducats, 1 MP.
Tier 2: 20 ducats, 2 MP.
Tier 3: 60 ducats, 5 MP.
Tier 4: 120 ducats, 5 MP.
Tier 5: 240 ducats, 5 MP.
Tier 6: 320 ducats, 8 MP.
Tier 7: 640 ducats, 12 MP.
Also increase the benefits, especially for the higher level buildings. Make them influence non-monetary and non-manpower aspects. Just like the Counting House (but stronger - 100% Counting Houses should reduce inflation by 0.30% per year). Have Diplomatic buildings which reduce war exhaustion (-0.05 per year at 100% coverage), have Military buildings which increase Army Tradition (+1 army tradition at 100% coverage), etc.