Polythene is injection moulded so you will need steel/metal moulds which is where the costs rank up.
Not really, where I work we have experimented with aluminium molds. PE can be processed at 140°C (altough we usualy go for 160 to 180° and in rare occassions even 200°C)
The biggest danger lies in the clamping force. Too high and you'll deform your mold (Yes, even a good steel mold gets pressed together a tiny bit from excessive force)
The closing speed is another thing to watch out for. In plastics manufacturing cycles make up the cost. (time is money)
Lousy technicians try to skim tenths of seconds off a cycle by augmenting the closing speeds. In the end they damage the parting lines and flatten the ventings of the mold

The clamping force to be used depends on the projected surface that needs to be clamped (basic physics) so If there aren't too much figures in the mold, it could be kept low.
Nevertheless, keep in mind that the creation of a mold is rather pricey. We got (large) molds costing more than a high end Merc, Beamer or Audi...