You are right the economy of scale should bring the cost down. In my current community the builder formed the community and at the first AGM 300 euros per house was agreed between the builder and the administrator in Feb 2002, as each house was purchased this cost moved to the house owner and the builder got his 300 back. This worked very well, because if 10 of 15 houses are sold for example the builder still owns 5 of them so he has to pay.
We took possesion of the house in March 2002 and in July 2002 we had an EGM because the administator was a pal of the builder and he was slow to fix issues. At the EGM we invited a new administrator and we/she agreed to switch and she took control. We set a new budget of 700 euros, it now 900 plus 300 for a painting fund. We have had this administrator for 3 plus years and they are very good. During this time we had a number of 1000 euro plus projects for improvements all paid out of the fund with no additional cost. This year we added a one payment of 450 euros per house to put pool heating in. With our budget we set it 20% higher and give 20% discount for payment within 30 days (this virtually eliminated debt !!) and we split the payment in two 6 month bills.
The start up of any community is (should be) the most difficult time, it then settles down.
You can call a an EGM at time as required......have a look at the Horizontal Property Act, I have inserted the clause below from Section 16 of the Act
(1) The meeting of the general assembly shall be held at least once a year to approve the budget and the accounts, and in any other time the president sees fit or upon request of 25 percent of the unit owners or a number representing, at least, 25 percent of the assessment quotas.
At this site and many others on the web.
http://www.eyeonspain.com/spain-magazine/horizontal-laws.pdf#search=%22horizontal%20property%20act%20english%22
The community set their own budget and agree accounts, but you need to cover all costs, must no got into debt, and establish a reserve fund. Read the Act when you cannot sleep is only about 18 pages.
As a member you can ask to see a copy of the accounts. As President of my community I get an email each with the accounts and I check for any anomolies and I offer the accounts to the other members.
Hope this helps. Jeff