Properly installed, a Firestone Liner will virtually last a lifetime. Last I checked, they had a 20 year guarantee. My sister has a small pond that is in the 14 year range, unprotected, and my dog use to go in it. Holding up so far.
Silting is probably going to depend on your soils and topography/runoff. Not qualified to answer that one, maybe someone else will fill in the blanks.
The best price I have found for a 50 x 150 Firestone 45 mil liner was around 1600.00 out of Kentucky. Shipping would be extra. You will also have to get all the stuff to seam the pieces up. I am probably really low balling it at 7K to 8K if you do all the work yourself, have equipment and plenty of clean soil for under and over the liner. You also have to install vents to atmosphere under the liner or you could end up with a giant balloon due to gasses from under the liner. I think they call them Whales.
I am a bit rusty on liners right now, so if anyone wants to make corrections, add comments..., please, feel free!
A liner could be the best thing you ever did, or a total disaster.
Actually, The only thing I am qualified to answer is, Don't stand barefoot in water and try stick your fingers in an electrical outlet