I think that asking $60 for a multiplayer only title is perfectly acceptable UNLESS there is a season pass to go with it. That aggressive pricing model doesn't seem right to me. Now, if they want to charge $40 for the base version and $70 for the complete version which will have content that's rolled out over the course of a year, that's fine with me. But launching a full priced game that is marketed as incomplete shouldn't be the norm.
I honestly don't see any issue with charging $60 for a multiplayer only game. Just because it doesn't have SP, doesn't mean it didn't take a lot of money to develop and market.
If the content is there, the game will prove itself and sell for that $60, regardless of it's it is multiplayer centered or not. If not, price will drop in time accordingly. In fact, I would say multiplayer focused games ought to cost more than single player due to the costs of maintaining servers, balancing patches, continued development, etc. People don't realize that games are becoming bigger and bigger in scope and cost, so in all actuality, we're pretty lucky that games are still only $60 new as it is.
I'm the last person to buy a multiplayer-only game but I will admit that if we are OK to pay $60 for a single-player only experience, then I don't see why $60 for a multiplayer experience is an issue. I believe it does cost a ton to maintain servers and a continuous online service. But on the other hand, the stigma attached to multiplayer games is owing to the lack of content typically found in these games. A lot of multiplayer-only games have had too little to offer and as a result, they've earned a bit of a bad rep. Let's not forget that playing games online also cost an additional fee in terms of PS+ and Xbox Live subscriptions so the $60 price needs to be justified by the content on offer.
As someone who plays a LOT of multiplayer games, I'm definitely OK with this. Would I want it to be at $40-$45? Yes, I would. But the reality is, publishers and developers need to make money to stay afloat, and it's expensive to make a quality game regardless if it's just a story-driven affair, a multiplayer-only shootfest or a combination of both. Plus, there's servers, client patches and all sorts of post-release stuff that we don't even know of.
There's also the stigma that a game that's "only" priced at $40 isn't a AAA game even if it is, which is something I'm sure publishers want to avoid.
At the end of the day, though. I value a game based on how much fun I have with it, how many hours of enjoyment I squeeze out if rather than if there's a crappy story mode tacked on.
If a game was designed to be a multiplayer-centric title, then devs should be given the freedom to make it the best it can be and not forced to work on a single-player component just to have it as a bulletpoint on the back of the box.
When it comes to calculating the value of a game, I usually look at the hours of enjoyment it's brought me.
Multiplayer games, even without a single-player portion, have always given me value for my money, and so I have no problem putting down $60 for a solid multiplayer experience like Overwatch.