I never use the bikes, I also never use the pool.
I already have a full weight setup (includes 2 benches with attachments, power rack, full set of dumbells up to 45kg, full set of olympic plates roughly 400kg worth and a host of bars and other stuff such as kettlebells, etc), as well as a boxing setup (reaction ball, speed bag, hanging punch bag and freestanding kickboxing bag, etc). Look at the official home gym building thread for more info.
The only thing i would miss is the sauna but you can buy them for around £5K for a 6 people, wood burning so you don't need to worry about energy costs.
http://www.hottubsbarn.co.uk/saunas/traditional-saunas.asp
I also have 4 members of my family which use the equipment but only 3 of us who use it regularly.
I also have a gym membership, it works out roughly £125 a month just for me to go to the gym (£80+ in fuel, plus £43 monthly fee). This is purely so I can use the treadmills and sauna, nothing else.
So just for me it costs just under £1.5K a year to go to the gym.
I guess the main issue is that my gym is a 15-20 minute car journey away, which can become 30-45 mins during rush hour with it being located in the city centre and me staying out in the sticks. There are closer gyms but council run and they just aren't as good.
So it depends on your own situation, but if 3-4 of us were all paying gym membership you can quickly see that £1.5K a year turning into £4K+ very quickly, especially since we couldn't all sync our schedules to car share, etc.
I always buy decent stuff, so I know it will last. My weight equipment should last several lifetimes, so it can be used by future generations, etc also. Sure the initial costs are high but in the long run the benefits are too.