I've been scanning Ebay and Dragon Driving and it looks as though I could pick up a cart and harness for around £300 which isn't bad at all - about the same price as a decent secondhand saddle.
If you don't want anything really fancy or suitable for showing in hors concours driving classes, and if your pony is not that big, it should be possible to get something for under £500 easily. I am kind of half looking for a 4 wheel flatbed cart - its called a trolly in diving parliance - so that Sunny could haul logs up from the fields or take loads of stone to gateways and such like and be a generally useful kind of pony - like I know he wants to be!
I use a breast harness which is a mix of nylon webbing and plastic - its not that wonderful, but it does the job safely enough and requires no softening with saddle soap etc to keep it comfy and safe! I cant remember how much it cost, but I think about £200 including blinker bridle, though this was a few years ago. Some people believe (esp in USA) in breaking horses to drive without the use of a blinker bridle, on the basis that the horse should be OK without blinkers if he has been introduced to the whole idea in the right way. I think they may have a point actually, but I have always used a blinker bridle with all of ours and so have all of the owners I have driven for, so I suppose I am sticking with what I know.
Come over any weekend DM. Next Sat is actually fine for me. I am planning to start long reining young Hebog this week as its high time he learned this skill, ready for starting him with ridden work, so maybe he can do a demo too!
Email me if you like and we can set up a good time xx