Ok, then keep him. Congratulations. It doesn't change the fact that the objective value of goalies is far lower than d-men.
If there were such a thing as "objective value", goalies would never go for as little as they usually do, since it is an absolutely critical position on the team. Goalies tend to have less value, because "value" is completely subjective.
Here are factors as to why goalies have less value:
1. Difficult to project. Murray's career could range anywhere from Cam Ward to Ken Dryden (the latter being extremely unlikely). Nobody has really gotten good at evaluating goaltenders to determine how that will go.
2. There are a grand total of 60 (soon to be 62) NHL jobs for goaltenders out there. For which there are several hundred aspirants to those jobs. When the supply of goaltenders outpaces the demand for them, the price tends to go down.
3. Take a look at the names of the goaltenders that have changed hands over the last several years. The elite ones aren't moved in their primes. They're either moved before anyone knows what they'll really be, or they're moved after they've past their prime. Luongo could be argued as an exception here, but his toxic contract really messed up his value in trade.
Just saying, "well, goaltenders never get a big return" ignores that the goaltenders who would get a big return are never moved in the first place, so we don't know what their value would be. Do you really think the Canadiens would ever give up Carey Price for anything remotely similar to the goaltender trade deals we've seen, for example? How about the fact that Tampa Bay decided to hold on to Bishop this summer, even though they risk losing him as a UFA next summer? Seems like they value him at a price higher than goaltenders have generally been traded for so far, doesn't it?
All players have a value to the team that currently has them, and a value to the team that wants to acquire them. Trades only happen when the team that wants to acquire that player values them the same or higher than the team that currently has them. When they don't, the trade doesn't happen. There's no such thing as "objective value" or "value in a vacuum."