Assuming you are using it for RAO/COPD/SPAOPD/Heaves or whatever its current name is, then steroid inhalers are more effective than Ventipulmin and significantly cheaper. The only issue is whether your horse is co-operative enough to let you administer them properly.
Hi my boy is currently on steroid inhalers from the vet for his COPD, he started off on 20 puffs a day which meant a £40 inhaler only lasted for 10 days, but we are slowly reducing the puffs and he is currently on 14 a day and doing well and we are hoping to get it lower than that. The drug he has is Beclametasone 250mg and they cost £40 from my vet (200 puffs in each) but you can get them online for about £22 but you will need to get (and probably pay for) a prescription from your vet.
Thankfully my boy is a total saint with it, it does take a bit of practice to time it right so you puff as they breathe in, but I'm quite clumsy and I still manage!