It's a big ballpark, but without seeing the location or knowing what we would be working with, we would estimate a figure between 15-25k.
Materials and taste (tub/shower, tile, fixtures, cabinets, countertops, flooring, etc.) will influence the cost along with landscape, and location of mechanicals.
What is the in the space currently? What do you want the guest room/playroom to include?
Like AK mentioned this is a big park...foundation and terrain can influence this as well as "tie-ins" to the existing infrastructure...plumbing electric. If this were a simple "guest" room the price would be a good deal cheaper as the plumbing concerns could be avoided Baths can also eat a lot of the budget with fixtures fancy tile etc.
150 to $200 per sq foot is normal...but will vary from region to region