The Post has a story on how rebuilding the CCT with the Purple Line will cost about twice as much as previous planned, with the headline "Costs rise to rebuild Capital Crescent Trail along proposed Purple Line route" which makes it seem that the cost of the trail is just going up. Only way down in the text do you find out that one reason why the cost has gone up is that the county wants to make the trail better than they had previously planned.
Some of the higher costs stemmed from county requests to widen the trail from 10 to 12 feet to provide better emergency vehicle access and add lighting, officials said.
So yeah, when you decide to buy something better than what you had budgeted for, it will usually cost more. Furthermore
Berliner and Erenrich said county officials considered the previous trail estimate to be a budget placeholder that would grow as engineering progressed. The MTA has now designed about 30 percent of the project.