upgrade fully? anywhere from $200 to $600.
Here are the basics of upgrading:
To get range and accuracy; make sure your compression parts in the mechbox are sealing perfectly. Buy a high quality barrel and a good hop rubber. Done.
barrel group will be around $90-140
compression parts anywhere from $0 to $100
Everything else just alters reliability, rate of fire, and trigger response.
The most common failure of any gun is the piston, $15 gets you an SHS piston that will last the life of the gun.
Generally, just replace parts as they break. Upgradability means the gun is compatible with aftermarket parts. This inherently means that the stock parts will work with any aftermarket parts. So you don't need to upgrade the whole thing all at once.
But the very first upgrade you should do, above all else, is just put a decent hop rubber in the gun. It's the cheapest upgrade possible ($10-$20), you can do it yourself, and it will make the biggest difference in range and accuracy of any upgrade.