Money for a new one!
Unless its already been modified with aftermarket parts, the only thing you can really do is get a price that will leave you money for a replacement. Under $1500 for a fully splined rebuilt or around $2,000 for a new one with a fully splined shaft (if you know where to look).
I would definitely drive in the upper end of the RPM range and stay away from applying full power or lugging at around 1800 RPM. I lost the 5th gear nut twice before it the transmission was replaced with the fix that preceded the fully splined mainshaft. That transmission was replaced 10 months ago when a the 5th gear syncro came apart. Two of those failures occured on trips where I was hauling the camper and probably did a little to much driving at the lower RPMS. Just my opionion.
When my 5th gear failed the first time only a few people had even heard of the failure. And the fact that Dodge has never implemented a working fix, even though the gentlemen who holds the patent on the fully splined shafts has made great efforts to get them to, is one of the many reasons I will probably not buy another Dodge, at least not a new one.