Putting a device into space is relatively cheap for a project like this, and is only dropping with each year. Say $100M - $200M, on a $10B project, puts the launch at just 1-2% of the price tag. I believe the Hubble total project cost is pushing past $16B in adjusted dollars.

It's the massive capital expenditure on the telescope and its infrastructure that is the significant limiting factor on the pace of putting newer and better telescopes in place.