135 isnt a bad price, less then 35 each. They are better then your average tyre, pretty good. You can get toyo T1R for similar money (about 32-33 each i believe) I personally would go for the toyo, but the yoko would probably last a little longer.
i presume you mean 195/45R15, and that prices sounds about right, they are normally amount 10 more then 195/50R15 proxes so you should be looking around 45 each
I've been looking around for Proxes for mine and the best place i found was www.camskill.co.uk. Worked out to be £32.10 a corner delivered (think there's a price limit on free delivery though, i did it for 2 tyres and was free). I'm in a dealership too and i can get them for about £37 i think. Blackcircle's want just over £37 plus delivery.