I know people like Eirik here, but him not starting over Saunders for most of the season says a lot about what Vieira thinks of his talent. I think we were always planning to upgrade at keeper this off-season. With that being said, I don't think Johnson's acquisition cost is that bad. His salary is $100,000 higher than Saunders' (who I'm assuming will be gone) and we spent some TAM and GAM on him (let's go with the $100,000 that's been floated here). So $350,000 minus $150,000 leaves a $200,000 investment to upgrade with Johnson. Not using an international slot on him should also be factored in his costs, though, assuming our upgrade alternatives were international like they've mostly been for us up to this point. If we don't plan to go over our international slot limit this move gives us flexibility to get international players at other positions. If we do plan to go over (probably more likely), then getting him saved us the TAM or GAM ($50,000 assuming what has been discussed is true) that getting another slot would cost for an international upgrade. That means the upgrade from Saunders to Johnson cost us roughly $150,000.
Now lets look at what that upgrade cost gets us. Johnson has a reputation as a great shot blocker who had a lot of potential. At minimum he should be a decent upgrade on Saunders (and likely on Eirik, who Vieira did not play over Saunders) and if he hits his potential a significant one. The one big holdup seems to be that he hasn't played out of the back much. Remember, though, that we spent most of last year using Saunders to play out of the back. Johnson probably can't be worse than that, and will learn as time goes on.
Overall I think we needed an upgrade at keeper. I also think it would have been tough to find an international upgrade for the $150,000 upgrade cost that Johnson is over Saunders. I think Johnson will be well worth the investment we made, especially when MLS continues to increase the amount we can spend.