You can pay an absolute fortune for heated propagators. I have had several over the past four decades and I have just decided to buy some new ones this winter. In the end I have settled on the Stewart 38cm propagator and added remote thermostats. The Stewart stuff seems well-made and durable. When I bought them a few weeks ago, B&Q were selling them for £15.00 each, I see today they are now over £20.00... Time of the year I guess, still good value though. There are all sorts of remote thermostats available (A Google search will bring up lots of them) and expect to pay around £15.00 each. This way for around £35.00 (at today's prices) that has to be a good value. I have also bought a couple of heated mats that can be used under non-heated propagators for versatility.
Just a mention regarding use of the temperature probes on the thermostats. There seems to be some debate about where to place them. Some say they should be in the soil, others say they should be in free air. Well, in my humble opinion, both are correct. It depends on whether you are maintaining soil temperature for seed germination or maintaining air temperature for growing plants on.
The power ratings of the thermostats are high enough to run several heated propagators form one unit. However, they will all run at the same temperature which, if you are growing lots of similar plants, is not a problem. Having more than one thermostat will give more flexibility.
I hope this helps.
Ralph.