I have a very tall and keen 7 year old who wants to start skating? What is a good (and easy to master) starter board? Roughly how much should we expect to pay?

you can get pretty respectable completes from online shops like Route One or London’s famous Slam City Skates. I’d go for something about 7.5″ wide at his age, maybe 7.75″. I was going to say £100 or thereabouts but you could go a lot cheaper – £80 or £90 for a branded deck and some generic trucks and wheels. Google complete skateboards.

£50 ish mark you can get a Renner C series or Enuff complete. Fracture aren’t too bad either.
Not boutique, but serviceable – I use Enuff wheels on some sale trucks with a Krooked deck I got in a sale. Should be good enough to let him upgrade bit by bit (good wheels and bearings first, good deck with more pop next.. not really noticed much difference in trucks other than some breaking kingpins more easily than others, and weight).

I got my boy one from SS20 in Oxford. One of the original old school shops. I was advised on one for about £70. It was scaled down a bit, so not quite full size. Modern boards are smaller now anyway, almost like the old freestyle boards with a double tail (front and back).
They did advise me to get some decent rubber bushings, that are a bit softer. I got independent ones.