Charlotte is marrying a man she has known for less than four days (they met for the first time at the Netherfield Ball, she sees them the next day, and the day after, Mr Collins proposes first thing the next morning). If a friend of yours stated that they were engaged a man they had known for only four days, would you regard them as prudent?

Now let us imagine that your friend lives in a time where divorce was nearly impossible, where the jurist William Blackstone stated:
By marriage, the husband and wife are one person in the law: that is, the very being or legal existence of the woman is suspended during the marriage, or at least is incorporated and consolidated into that of the husband: under whose wing, protection, and cover, she performs every thing. (Commentaries on English Law (1765-69))
This is a world where a woman's happiness and perhaps even her life is incredibly dependent on her husband's character.

Oh, also, if you died in childbirth your husband would have the job of bringing up your children. It is little wonder that Elizabeth says:
As yet, she cannot even be certain of the degree of her own regard, nor of its reasonableness. She has known him only a fortnight. She danced four dances with him at Meryton; she saw him one morning at his own house, and has since dined in company with him four times. This is not quite enough to make her understand his character." (chpt 5).
Charlotte is marrying a man she has only known four days, and has not known by character beforehand (except for Elizabeth pointing out his oddities), so I can't regard her actions as prudent or as being taken with open eyes.

And, furthermore, as you say, she lives very close to the Bennets, whose unhappy marriage appears not to be 100% in the kids' favour, but we never hear of Charlotte thinking what Mr Collins would be like as a father.

What I think shocks Elizabeth so much about Charlotte's actions is that Charlotte is placing all her trust of future happiness in having enough wealth. I agree that it was regarded as important at the time for a couple to have sufficient fortune between them, what was wrong about what Charlotte did was that she *only* cares about fortune. It's like buying a jacket by fit and ignoring whether it will keep you warm or what colour it is. Charlotte made her decision based on one factor, unlike what Elizabeth advocates of multiple ones degree or her own regard, or its reasonableness.

Charlotte deciding to marry a man *just* for the money is like putting your mortgage payment down at the horseraces. She may be lucky and it may all work out, but it isn't what I would describe as prudent.