To elaborate a bit:
1. Costs go up the more you buy. The big increases are at the infra jump points, so make sure that you read up on those before you hit 1k infra.
2. Marble, lumber, iron and aluminium will all reduce the cost of infra, as will building factories.
3. The harbour is the most important, which you have. You don't need the school or police HQ yet, after the harbour and foreign ministry it's generally accepted that the next improvements should be 5 factories, 5 banks and 5 stadiums (although the order varies).
You have the rest.