How long are you willing to wait to find an already raised axle
trailer? That's the question you need to consider. By your own admission you said you had never seen one for sale!
The raised axle is an option on these 5th wheel trailers. If you add it yourself or find a 5th wheel trailer that meets you needs already done makes no difference. Adding the raised axle yourself lets you have piece of mind that you have a new axle, new rubber if a "Torsion" rubber reflex axle, new brakes
and a proper axle alignment. Buying a trailer already equipped with the raised axle means you must review the condition of all those items I have listed above.
I would buy the best trailer you can find in your budget without regard to the axle. I would know up front my cost of installation of a raised axle before I purchased anything. Personally I would ADD the raised axle to the best trailer I could find if not so equipped. Yes that's an extra expense for you. If you did find a trailer with a raised axle already installed I think you would pay $0.00 premium for that axle as this very well could make the trailer less desirable to many buyer's.
A seller of an already raised axle trailer may "think" a raised axle on their trailer is worth a premium however many buyer's may have difficulty matching the trailer with the raised axle to their tow vehicle do to the additional trailer tongue height. There is a reason you see very few if any raised axle 5th wheels for sale
,on the road or at camp grounds.. Most folks do not need it!
Buy the best 5th wheel you can afford and then make it your own by adding a raised axle if that's what you really need!