Argos prices go up and down all the time, but I've been keeping my eye on the character nightwear as they often reduce it to really bargain prices. Right now they have Spiderman*, Superman* or Buzz Lightyear* Pyjamas for £5.99, or the Thomas & Friends* ones are £5.94.
The sizes vary according to the design between 18 months and 8 years.
As always with Argos stock levels are not guaranteed, but some of these look to widely available, especially the superheroes, and depending on your location can be delivered as well as collected from store.
The Spiderman* and Superman* ones look particularly good as they can be used for dressing up as well as nightwear - the Superman PJs come with a detachable cape, and the Spidey ones with a mask so your boys can run around being superheroes tiring themselves out nicely before bedtime.
There are also a couple of sets of Moshi Monsters* and Skylanders Giants Pyjama*s which have TWO pairs for only £6.49, but these appear to have much less stock around. Might be worth checking your local stores for stock if your kids are fans of these as they are an even better bargain.