It really depends. As a Toyota tech, I would recommend taking it to a Toyota dealership. They know exactly what to do and how to do it, rather than an outside shop, who sees maybe one or two 4Runners a year, and is quite possibly learning on your truck.
I would say, bring it in and start with a leakdown test. They may surprise you and tell you it's a quick and simple fix.
If it is the head gasket, it could be anywhere between 400 and 800 dollars.
And don't be fooled thinking outside shops are cheaper. Remember, they're independantly owned, and they could really use the cash. Dealerships get money from new and used car sales, parts and service(plus the owners are usually into real estate and the stock market). Average hourly rates at an outside shop are around $90, meanwhile the dealership is probably around $98. On a 5 hour job that's only $40 more, well worth it IMO for factory trained technicians and OEM parts.