Black powder is very safe, if used with respect and all rules are followed as with any flamable or explosive material. Have you ever heard of a Banger, or any other firework spontaniously going off without any outside source of fire or heat ?, they use black powder in them (plus different elements to make all those pretty colours in the sky ), but seriously, it is as safe as any other BP propellant. I know as you have said, BP is cheaper than Pyrodex and Triple 7 for that matter, but have you tried cleaning a BP revolver that's had black powder used in it, compared to one that's used Pyrodex or Triple 7 ?
The gun that's had Pyrodex used in it is far more easier to clean and one that's used Triple 7, is even easier to clean than the Pyrodex, plus they are much less corrosive than black powder, so cost is a small thing to weigh up against the advantages of using the other two, also as mentioned earlier, you don't need a licence to buy it, or a transport licence, or a special box to keep it in. I've found only one or two people in our club use black powder, but they are real traditionalists, who also do the re-enactment battles etc.
But as said, black powder is safe, but it has a few disadvantages, compared to the alternative propellants, but try both, I'm sure someone at your club will let you have enough black powder to do a card or two, then clean your gun and have a go with some Pyrodex, or Triple 7 and make your own mind up. Think of how much your licences and box will cost and how long a tub of each of the propellants you would use costs over a year or so and a lot depends on how much in each charge you use, when adding it all up.
Anyway, black powder is safe, it will not go off if you drop your flask (although, it's better to have all of your charges pre measured, in little phials, vials, test tubes, whatever you want to call them, before you get to the range, makes loading quicker, easier and above all safer, as it's less to go off, if there is an accident of some kind !!!)
Probably most, if not all reading this, will think, what a load of Bo((ocks, but it's what I've discovered in my reletively short time on the dark side, so I say it as I see it. Hope some of this was of some use

All the best
Baz