I have an incorporated company with general liability and errors and omissions insurance. No claims ever. A few years ago, with a 1,000,000 limit each, I was paying ~$1000 a year. Then a client need GL @ 2,000,000 so premium jumped to ~$1400. Then the same client needed GL @ 5,000,000 so premium jumped to ~$4000 (this client has a blanket policy that all contractors/consultants need at least 5 to work with them regardless of how small the risk is for a specific contractor/consultant). I think I am hitting minimum policy limits so insurance is disproportionately expensive. You will have the same problem. Your revenue (and their exposure) could increase by an order of magnitude and your premium would likely be unchanged.
As for WSIB, setting it up when you don't have employees isn't as easy as it should be. You need a client to force you to get it and then they will let you setup a voluntary WSIB account. You have to prefund it with ~$250. You report quarterly and they draw from that retainer. I set it up with the minimum possible income (~$30K IIRC) as I don't care about the security, I just needed the clearance certificate. If I were you and clients weren't forcing you to provide a clearance certificate, I would not bother with WSIB.
I understand the appeal of working for yourself, but for two days of revenue a month, the administration cost is quite high. Presumably you have been around the field for a long time, do you have someone you could wash this through? You bring the client to Company A, you do the work, company A issues the invoice and collects the money and pays you as an employee. You are in a slightly shady area of contractor vs employee, but if you structure it properly it should work (eg. Company A sends you an email letting you know that you need to work at site X at a specific date and time, your hourly rate would be a number, but the number would be derived as a % of the rate billed to the customer.)
EDIT:
Based on MM's price, maybe I need to get some more insurance quotes again. What a pain in the %^@.