When it comes to web development I find that this is useful for people to have a visceral feel for when they should be making changes and what is involved.
Wireframes are like working with post it notes, quick to move around and get a feel for the general idea.
Photoshop designs are like working with thick card a bit more rigid and a little annoying to cut but and the quality is a bit better and you can get a better feel for the final site, whilst still being relatively light.
Development is like carving stone tablets. Once there is code behind it those aren’t cards, they are heavy and hard to move. They are also interlinked and connected to the other tablets around them. If you want to move the tablets you need to rewire the plumbing and electrical cables. If you want to change the design you need a chisel and if you make too many changes then you destroy the tablet and need to get a new one.
At each step the amount of time and work involved probably triples. That is, 1 hour worth of changes to the wireframes will take about 3 hours in design but over 9 hours of development work.
The take away is ; if you want to make a change, do your best to work that out before the developers have had to carve out the stone tablets.