I prioritize any heirloom pieces with an xp bonus since… well… that’s the primary reason I use heirlooms.
If you bought them all it’s about 515,000 gold.
Personally I wouldn’t do weapon upgrades, at WoD the crafted ones can be obtained at level 91 and fully upgraded to serve their purpose if you choose. Legion gives you a weapon at the earliest level 98. BFA crafted weapons when upgraded are ilevel 233 at level 111 which can hold you off for a while.