You must understand that there is a deal of difference between a Stuart Turner and a Mamod. The former is most probably built from castings by an accomplished model engineer, the latter is almost a toy. This does not denigrate Mamod steam engines, as they are excellent at what they do but the difference is mainly in the fact that a properly engineered steam engine involves lathe and milling work, possibly taking months of dedicated work to fabricate a working engine. The Mamod comes as a working artifact with only purchase involved. This is the two sides of  the coin, do you want a nice running steam engine that you can buy in the shop, or do you want to spend quite a few hours  of intricate metal fashioning to achieve a fine working steam engine? This may explain the price difference between the two.

Cheers, Gerry.