I run my own business and I am always suspicious when someone can't give a straight cost answer! So yes, be up front. If you are ridiculously expensive then people won't join no matter how great the instuction.
But... (!) on the other hand I wouldn't go into every single cost breakdown either, but rather "It's £5 an hour but your 1st lesson's free" kind of thing. If you start going on about joining fees, insurance fees, the cost of buying a gi, etc etc. then you'd start to detract from the training you are offering. Simple and straight always the best!
Unfortunately there will always be people who's sole determining factor will be cost, so if you're £5 an hour and a competitor down the road is £4.50 an hour they will go to him regardless of what kind of teacher he is. That's life!