For any other cheapskates out there I'll just mention that it's not too hard to create your own tungsten carbide spiked shoes. What you need: spikes from supatracks.com (sold out of shoe sized ones last week though), suitable shoes, best in harder rubber and a supportive stud, mine are in Salomon speedcrosses, old Inov8s the grey ones with quite big edge studs look good. Carbide spikes 50p each, I used 10 per shoe.
I drilled pilot holes and screwed the studs in with a bit of isocyanate glue (rally cars use this technique, the glue is a belt and braces approach), ten minute job. Few long mixed terrain runs now and very successful, all studs still present and correct .
They'll not work well in small studs in sticky rubber.