Help looking for an AC power transformer
For ANY primary voltage (except 12V!) and 12V secondary, 30A would require a 360W transformer, and for a resistive load that means 360VA.

At 50Hz or 60Hz, that is a substantial quantity of copper and iron, so the material costs alone impose a lower limit on the possible price.

Switching power supplies eliminate this lower limit by driving much smaller and lighter transformers much faster, and thus eliminating the basic need for large quantities of raw materials. 

All else is detail; but it is certainly feasible to make a big SMPS much lower cost than a big 50Hz transformer.

For a one-off project, a transformer is a simple solution, and you may be able to find a "surplus" bargain if the "proper channels" are too expensive. 

Alternatively you may find a "gamer" PC power supply with 30A or more available from a 12V DC rail; the commodity nature of PCs helps to keep prices low.