The cheapest method is to isolate spores from the soil by the wet sieving and decanting method of Gerdemann and Nicolson (1963), I believe. You only need sieves, water, Petri dishes and a microscope. Of course, if you have a centrifuge hanging glasses to centrifuge the soil that is in the sieve with a concentrated solution of sucrose and obtain the cleanest spores, it is ideal
It depends on what you mean by 'mycorrhizal spores' - ecto - and endomycorrihizal fungi are completely diffferent things. Eduardo's suggestion sounds pretty good for the endos.