Down and back about 5 years ago. I'm not nearly as budget-oriented as I used to be, so take with a grain of salt: US$2000 for upgrades, luggage and riding/protective gear. $2000 shipping n/i shipping the bike home afterward. $1500 for flights and/or boats for yourself, including homeward journey. $1000 for consumable parts like tires, cables, brake pads, seals, oil and filters. That's almost half your budget.
Camping isn't practical along much of your route, although it's certainly possible for the truly dedicated. And some costs depend on the specific route you choose--Brazil, the Guyanas, and Chile are expensive in general, but Chile's got an established camping culture while the others don't.
Another approach: 8 months is about 240 days. Leave home with US$10,000 and you've got $40 per day to play with. In some countries that won't pay for your fuel if you're moving along. Maybe you'll try to avoid those countries, or maybe you'll be staying in place more than I. But staying put tends to cost money in its own ways.
On a positive note, glad you're allowing enough time for the trip. Many don't.
All of this has been discussed a lot on this site. Do a search and settle in for some entertaining reading...but remember that grain of salt.
Mark
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