I took my son on this ABD last summer and I think $300 per person is reasonable too. This includes staying one pre-night in Hollywood and 2 post nights at the GC. Some of the expenditures included $300 for tips for the guides, $60 for a shirt and hat at Henson (they really don't have much - its an ad hoc set up on a table), only spent about $30 for a shirt at the Studios (they have a lot but not much unique stuff - the shirt had the Studios logo - I think they also took the D23 discount if you show your membership card), and about $30 at MOG (also for a polo shirt - ABD gives each person a voucher for 10% off one item - when we went they did not have much in limited editions or pins).
We also spent about $300 in food - $120 at Napa Rose, $60 at Steakhouse 55, and $65 at Trader Sams, and about $35 each at Cafe Orleans and Pixar Pier ( lobster nachos). I think most of these places gave us a 10% discount with my DVC card. I would suggest looking at the restaurant menu's and reviews so that you can plan. To me, Steakhouse 55 was the absolute best - BUT (and this is disappointing) it looks like ABD is removing the breakfast at Steakhouse 55 and replacing it with a quick service for next years ABD's. I also spent about $100 on bottled water and diet cokes too.
On our pre-day, we went to the El Capitan to see the Incredibles 2 movie - seeing a movie there was amazing - cost was $56 for 2 premium tickets - included popcorn and a drink. I also spent about $30 for Uber ($10 each way to the Grove/Farmers Market - don't miss Bob's Donuts).
Hope this helps - if you have any questions let me know - enjoy the ABD - its awesome!!!!!