The same way you priced up the original work!
how much did you charge for original quantity of sockets?
If the job is already in progress & you have got all your materials sorted..
If you then need an extra trip to the wholesalers to pick up more kit this must also be built into your costs..
my original quotes would have included sufficient costings to allow for my time & fuel, organising & collecting materials..
Extra trips cuz I forget something are cost taken from my profit margin! :_|
But extra trips because the customer changes the spec would need to be covered by someone?... Not ME!
As a rough ball park figure I would typically add an extra 3/4 hour per additional accessory + materials..
obviously if you have a tricky cable run to reach an awkward accessory position this must also be accounted for.
But also if its adjacent to and existing point with only 2foot of cable needed then obviously can be lot cheaper.. maybe just the materials covered! marginal extra labour cost, complimentary as a gesture of goodwill in the hope of extra work!