What's the difference?
Not a Lot - using the words of a famous magician.
They are both garden buildings that additional living space, without the need for planning permission. You could argue that a garden room doesn't require insulation, double glazing and electrics, however in this day and age, it makes sense to add during the construction stage rather than after manufacture.
Garden rooms also tend to be more traditional style buildings with pitched roofs, and feather edge cladding, as opposed to the garden office which is more often based on a modern flat roof design.
It is possible to come in under budget for a garden room providing you can compromise on either size or spec. A fully insulated garden room at this price will be small - around the size of 2.4m x 2.4m. Alternatively if it's not for year round use, then a bigger size without the internal fitting out can get you up to 5m x 3m.
We would now consider this to be a mid range product. Expect to get a good sized garden room, with the groundwork for this price - suitable for usage all year round. Something that gives the wow factor will be around 5m x 3m
Those with bigger gardens may well have a bigger budget, and be looking for something at the highest level. Bearing in mind that these building can exceed £30k, you can still comfortably get a garden room with a slate roof, plastered walls and the groundwork for £30,000.