Plexi comes in 6 mm and lexan comes in 12mm . Lexan is a fortune here . Actually I bought a piece of 6mm plexiglass which was about 18” x 24” and it was $35 , and the guy told me it would normally be $70 if he had to cut off a sheet , but it was a piece left over from another cut.
Pretty sure a 4x8 1/2” sheet of Lexan was well over $1000 ten years ago . Hate to see it now
Interesting. Here (Seattle, USA), I can get either in 6 and 12mm (and lots of other) thicknesses. This is from TAP plastics, a west coast USA store chain but I think most major cities have something similar. A piece of 6mm, 12"x18" extruded acrylic (aka plexiglass) is $14.70 USD. $21.60 for cast acrylic (higher quality, better optical clarity), same size, thickness polycarbonate (aka lexan) is $20.85, for comparison. If find extruded acrylic to be perfectly serviceable for small projects. 12mm poly is, gulp 39.30.
Last time I looked, you can find similar deals for smallish pieces on ebay/amazon though you have to shop carefully as there is a very wide range of prices.
One thing that has somewhat spoiled me is that TAP Plastics has an offcut box - each piece is $1. Some of which are fairly big (12x18-ish isn't rare). I've gone in several times to buy a specific sized piece and found something that would work in the bin. Saves $$$. Helps if you have a decent blade for plastic in your table saw so you can trim it down.
Edit: by the way, I used 4.5mm (sold as 3/16", measured thickness is 4.4mm) extruded acrylic in the planetary gear project. cheaper and has plenty of rigidity for this application.