This would be better posted in Arborist 101 or Commercial tree care & Climbing forums...
BUT, I'll pitch in.
Are you pruning or doing a removal?
If you're pruning, it is NOT recommended to use spikes, too much damage to the tree, debated ad nauseum on other forums. Get your climbing rope set nice and high and you can access the tree much better, get good support, and use your lanyard to help steady yourself.
If its a removal, go for it, spikes, the lot, just get it down safely. Climbing spikes are designed specially to penetrate and carry your weight without "gaffing out' when you stand up. Making your own could pose problems if you don't get the curve of the gaff right.
Its your safety, how much is it worth to you? I would think $200 was a fair price.