It really needs to be broken down into about 3 categories or tiers really.
Top category players (Players that are currently exceeding what we originally paid for them and/or are our best players):
Morelos - it's more or less anything north of £20 million. Highly dependent on clauses and sell on fees.
Barisic - if recent form persists then I could easily see mid level clubs across the big 4 in Europe taking an interest in him. That happens we can command anything above £6 million.
Tav - mid to low level English sides would offer a decent amount for him for a few good seasons. Depending on the window, he's in the same boat as Barisic. Possibly more due to the premium on English players down South.
Kamara - we paid 50k for him. We make anything close to the fees being touted for him we are winning big. He probably fits in and could swapped with Jack on my list but recent coverage suggest he's being watched with real interest.
Goldson - has looked imperious alongside Helander of late. We could double possibly treble what we originally paid if the English Market comes calling. Depends on the window. He's viewed as a key player by us.
Mid level category players (entirely dependent on development rates and form):
Jack - seems content with being a Rangers player. Underrated by fans but highly valued by the coaching staff. We'd be talking similar numbers as Kamara but I genuinely believe we have zero desire to cash in on him.
Helander - we would make what we paid for him back at the moment. He wins things with us we'd be talking of adding a few million more on top of that. Right now we'd make what we paid for him but likely slightly less.
Kent - biggest outlay of the current squad. We add more of an end product and goal return to his game. We are doubling his worth, if not talking Alfie numbers for him in a few seasons. Currently would struggle to recoup our outlay for him.
Aribo - has had an up and down time of it. He starts reproducing his early season form then we are talking what we paid for Kent with scope to charge more in the future. Very much depends on his development. A couple of million is his current market value.
Katic - again a similar idea as Kent and Aribo. We didn't pay a lot for him, we could potentially make millions on him. Right now he's worth about £2 to 3 million. He's one for the future.
Low level category players (players that have potential but currently aren't featuring much):
Edmundson - I'll preface this one by saying I rate this fella highly. He just hasn't featured an awful lot for us. Could be talking millions in the future. Currently we'd possibly just about scrape a million for him with maybe clauses added to that.
Docherty - hasn't really lived up to expectations. Don't see that changing barring an injury crisis in midfield and then again he would have to thrive for us. As it stands we'd make about a million on him. Would have made more if we had sold him after his season with Shrewsbury.
Polster - neither potential or experienced but again hasn't featured much. Could see him being sold back to the States for just under half a million.
Jones - has potential, still relatively young. I could easily see us making a few million off of him if an English side came sniffing.
Grezda - bags of potential but the boy is an idiot. Sadly a busted flush we will struggle to make any money on.
Foderingham - we let his contract run down. He's worth at least a million but sadly likely to leave for nothing.
Barker - just in the door, I highly doubt we'd make much on him at the moment. Has potential but has been out in the wilderness for a while though.
Stewart - not really a player I see being a massive or key player for us, therefore the potential to expose his ability is massively reduced and I just don't see any club down south showing an interest after his failure at Birmingham. Will no doubt be a decent squad player for us.
I would add a sub category to all of this. Players with no resale value or potential and low running contracts. Doesn't mean they are bad players, simply means clubs won't pay top money for them. McGregor, Davis and Murphy all in the too old category. Arfield could be sold for a decent fee but not on current form, plus for considerably less due to age as well. Halliday, Flanagan and Firth, all squad players with less than a year on their deals left but I don't see us renewing.