if you want to justify the expense of upgrades, I would put the value on what the upgrades mean to you, not some future buyer
in general, a bike is worth what someone will pay for it. so, one guy, one day may offer you $X for the bike w/o upgrades, but another guy, another day may offer you the same $X with the upgrades
also, wondering why it matters to you. upgrade your bike if you want your bike to have those upgrades. or maybe buy a better bike?