Easiest / cheapest way to build a dc power supply in the 0 - 50V DC range?
First, DC transformers don't exist. Transformers take an AC signal and can transform the voltage that way. A basic transformer will need at least a bridge rectifier and probably a capacitor at the load to give you something that resembles DC. 

You can almost always get a transformer that will meet your needs. For example, this transformer would get you to over 60V with no load, and still supply you with enough current to get your 0.5A current. Keep in mind that the voltages that these transformers are rated at are Volts AC RMS, which means that the peak voltage will be sqrt(2) times larger. There are lots of transformers available, you just need to look at someone's catalog (e.g. Digikey, Mouser...) who actually has a wide selection. Make sure that you get a transformer rated for the load you are drawing, and then add a safety margin.

Don't make your own transformer, especially if you are hooking it up to mains voltages.

Buying several transformers and putting the secondaries in series is a perfectly fine way to get your target voltage, but as you suspect it may be a bit more expensive. I don't think there is any impact on reliability as long as you're making good connections between the transformers. 

I would recommend against doing this, and if you do, make sure you have a fuse and a switch in series to give yourself some safety with your circuit. If you keep increasing the voltage, you will eventually exceed some component's breakdown voltage.