I don't worm mine but did have to worm one of my fiance's sister's mice once. I bought granofen from the vet - a small packet cost me 56p about 5 years ago but haven't used it again since. I think that packet had enough for 10 or more doses.
I deworm all incoming mice. I use ivermectin (also used to treat mice) or Stronghold (selemectine, this is a bit safer than ivermectin). In the past I've used levamisole, but than you have to treat them twice.
Because I'm a vetstudent, I can check for worms myself. I do routine check-ups every 2 months. I've never had to treat them, since I started deworming all incoming mice.
You have to be a bit careful with that. People here suck up (?) everything in a syringe with a very tiny needle. That way you can put a minuscule drop in the neck.
Being a vetstudent gives me another advantage. I can get hold of migycol (also available at your chemist, in holland the don't sell it to customers, but here they're stricted).
If you can get hold of it, it actually must cheaper! Put the Stonghold in a small bottle and diluted with migycol to 6 mg/ml.
Dose is 6 mg/kg, which means you can dose exactly right.
I was horrified once while cleaning cages and saw a roundworm; I have been treating my mousies regularly ever since. I'm glad to get new info, as I, too would like to dilute the form of of treatment I use (Revolution for Kittens and Puppies, another form of selamectin). I use a syringe without a needle for adults and a toothpick for the little meeces. Does anybody have any knowledge of transmission through either feeds or bedding materials? I have routinely frozen the grain that I buy from a grain elevator, and I'm wondering if Kaytee Aspen has ever been found to be contaminated? I always examine the packaging carefully to be sure that the vacuum packaging is intact.