I too feel that Charlotte was sensible in accepting Mr Collins, though if I were Marianne of S&S I would say it offends my romantic sensibilities :) . She has no independent fortune and her family do not appear to be wealthy, and they are numerous, so whatever money there is in the family has to be much divided. She is 'plain', is not spoken of as being 'highly accomplished', is not of high rank, and is getting old. What would be her future options ?? If she was accomplished she might be able to work as a governess or similar role; otherwise she has to rely on being supported by her family and ultimately by her brothers, or sisters if they marry well. She's actually quite vunerable to poverty, in either senario, of which I am sure she was quite aware.

Mr Collin's proposal enables her to escape from an uncertain future. He may not be pleasant, but he is probably not unkind or cruel; and also he's quite easily manipulated, of which she might also be aware - it didn't take a great deal of 'scheming' on her part before he comes to see her as a prospective partner for life, and this malubility might also make for a good husband !! :)