I think most would agree with your sentiments somewhat, however the situation changes when an attempt is made to put forward a suitable candidate for the same money.
Don't you believe the club have already been through this process & arrived at Jonny Evans?
I do see that side of the argument, and obviously I trust the club when it comes to player evaluation. There's a reason I'm just a lowly armchair manager.
If there is only 30m available and not a penny more, then I could understand why Evans is an option. Two alternatives that have been discussed around that price are Inigo Martinez and Ryan Bertrand. I won't pretend I've seen much of Martinez other than what I've read on here, and obviously Bertrand is a LB as opposed to CB (although I think LB is more of a need at this point), so from that standpoint they are imperfect alternatives. I don't think the decision is binary, though. I think they could go beyond 30m for the right player and in my opinion that's how the funds would be better allocated.
Another argument I'd make is - do we even need him at this point in the season? When the rumors surfaced over the summer I was surprised, but not necessarily opposed. Stones and Otamendi were coming off inconsistent seasons, Mangala looked like he was gone, and Kompany was an injury concern. So much has changed since then. Unfortunately Vinny is still oft-injured, but Stones has improved dramatically and is one of the first names on the team sheet when fit (hopefully soon?!). Otamendi has been quietly outstanding this year. Mangala is still a landmine on Bluemoon but I think he's played well enough, and we certainly haven't suffered in the results column during his fill-in. Even Tosin has flashed in his cameos, and while I"m not sure he's 1st team ready, I think we're nearing a point in his development where the club needs to decide whether it's worth giving him a shot.
At this point, the league is all but wrapped up barring an epic collapse, and we're in the knockout stages of the Champions League. We don't need Evans to hold on to the league, and he's certainly not leading us to Champions league glory. So what's the point of signing him at all in January? He just seems like stop-gap cover that we don't need. I think it'd be better to re-evaluate the position over the summer and work to identify a longer-term solution.
Apologies for the long post. It's 830 AM here and I haven't had my coffee yet so struggling a bit to get my thoughts down.