what is the most common or standard width of an internal door?

[jollygreen]

or more specifically, what size is most easily available (and therefore cheapest?)

ta
JG

[thundercrack]

what size is your door frame? most doors are 30" which allows furniture to be moved in or out. the height is standard whatever the width.

[Buenchico]

Just to support JG's post:
All of the interior doors in the current Wickes catalogue are 78" high and 30" wide.

Chris

[buildersmate]

Although you will only find a small range of sizes at places like Homebase (and Wickes to an extent), there are a range of sizes that somewhere like Jewson can get for you.
In common with the UK attitute to metrication on many other products, doors continue to be made in both metric and imperial sizes.
Imperial sizes are all 6ft6in tall and come in 3inch multiples of all sizes (widths) from 1ft6in to 2ft9in. 2ft6in is by far the most common.
Metric sizes are all 2040mm tall (don't ask me why), which is taller than imperial, and come in std widths 525, 626, 726, 826 and 926mm wide.
Regarding pricing, there are massive margins against so-called list prices. If you are buying more than about 3 doors, worth going to a Merchant for a quote when you should get at least 30% off list.
The 2 largest manufacturers are Premdor and Jeldwen, one of whom will (almost certainly) be supplying the big DIY sheds.