I don't see why it should cost you an arm and a leg. My shower is 1200x1500 and I am designing a seat into it so her indoors can sit and shave her toes... I could no find a
standard centre drain former at 1200 x 1500 so have gone for a 1200x1200 former and will use an extension piece using 20mm construction board. I am also going to extens it slightly to the side under the door so the tiles extend beyond the shower door. Deffo not standard. That plus 16 10mm construction boards (marmox) cost me less than £300. The nuance boards were the most costly bit at just under £100 each but they are cheaper than the tiles we wanted and for me much faster to install. If you decide to build your own shower base with sand and cement make sure you put fibres and a water reducing plasticiser in it to help avoid cracking.
As I said the whole thing, bear in mind for me this is a back to brick renovation...I forgot to include the cost of a new window in the list above... the shower is just under £2k which I think is pretty cheap.
buy aggressively, buy sensibly and don't skimp on quality e.g. use 18mm plywood for the base not 12mm as I was advised would be sufficient in topps tiles... LOL... I haven't bought anything from them in the end.