If you don't have a price for this listed on your website/profile then I would tell them it's full price even if a cut price BJ is something you'd think about offering in the future - guys asking these kind of questions are trying to haggle you down, they're too cheap to pay your going rate so they're seeing if they can persuade you to do it cheaper cos they'll be less work. Other varieties "I just want to wank while you watch" "I just want to give you pleasure" etc etc. What these men are doing is boundary pushing which means whether they genuinely do just want oral or whether they'll try to get away with more, they WILL be hard work. It also shows a bad attitude - I've got plenty of genuine clients who will pay my full rate for an hour just for a relaxing chat and a handjob or blowjob because they're respectful and want the proper experience, just not full sex.
There is nothing wrong with offering cheap quickies or reduced service sessions though, if it's your choice and you're happy with it - it does bring in clients and a lot of guys without much money to spend or who are nervous about meeting a new escort for the first time really appreciate having a cheap option to start out with. I offer a 15min handjob only session for ?15 which yes, is obscenely cheap, but most guys who see me for that end up coming back for a full service quickie or a longer session at some point, it's just a good ice breaker, and also on a quiet day I'd rather have a couple of guys with no money in and make ?15 off each of them than take nothing at all. But, that's a choice I've made that I'm happy with from a business point of view and from a personal point of view (anything that makes you feel devalued isn't worth doing) - I would never have offered it because a client suggested it because your rates and services should be entirely your call, and a client who thinks they have the right to influence how you run your business is a client not worth having.