I looked into this a few years ago, I don't have the paperwork on it right now, but if I remember correctly, a gauge and sending unit were around $50 US, this was 5 years ago .
The company is eGauges.com .
This is my queue to say that I am messing about with getting "copies" made of a BMW accessory instrument pod made in blow-mold plastic. I have a pod which cannot be satisfactorily repaired and I intend to use it as a model. The problem is that whilst I have access to a blow-molding plant, thermoset plastic has significantly less structural strength than the industrial resin the original article was made from which requires extensive redesign of the mold. The secondary problem may be that by the time I can get enough structural strength in thermoset, the finished article might look just a little "silly", or have no worthwhile service life. I was really hoping to use thermoset as a friend owns a thermoset blow mold plant (he makes bottles and jars) and would let me use his left over plastic prior to weekend shutdown to make as many pods as I can - especially if I do not care if they are clear, green, white or orange.
As a long term project I may cast up some in industrial resin but the minimum quantity of the stuff I can buy will make about 100 pods and if I put any allowance for time casting them one by one, the price will be not cheap whereas if i can get the thermoset idea working, they will be giveaways.