Having a two to three hour session with two nets of bowlers generally provides you with a number of major problems. With two nets, and 1.5 hours, bowlers (even with 6 per net) are generally tired at the end of 1.5 hours. If you run them for 3 hours, people will get bored.
The difficulty is that you can only ever have 3 or 4 batsmen at a time in nets (and most places don't get 3 or 4 nets) and if you have a bowling net with 6 or more people bowling, your bowlers bowl six, then get cold, then bowl six and pull something. Indoor nets just aren't set up with the space to operate properly.
I'd use somewhere like the Oval, book the entire hall, then have a machine net, a bowling net and the remainder of the space as fielding practice. When bowlers are not in their 3 over bowling spell, they are fielding, as are the batsmen. That way, you can keep all the people who turn up involved and doing something. Matches are a bit different because you do support the guys in the middle, or are on your toes when fielding. Nets just don't have that so keeping people going can be pretty tricky.
£5 per session is a pretty normal price at adult sub rates, with a discount if you pay for the lot in one hit. I'm not syaing it wouldn't work, but at ECB Premier level you are more likely to get attendees than at Surrey division four with that kind of set-up.