I've been through the very same issue myself so here's what i have to add:

I'm using an RVF400 (upside down fork) front end which has less offset than standard so that brings the wheel closer to the spark plug. The 150 cylinders are taller than standard and the big BR9ECM plug poking out the center makes things tight. I did however get away with it whilst i had a 120/60 Michelin Pilot Race on the front wheel. It was only when i put my wets on (120/70) that i started touching the plug cap when on the brakes.

The short RS plug and cap solve the problem immediately but as you say they're very dear (i get mine: Denso IAE01-32 from R S cycles in the US). I now run 120/70 size front tyres for both wet and dry.

You could try a lower profile tyre if you have a 110/70 (i assume you have the standard, narrower wheels?), but i don't know how easy it is to get a 110/60.
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