Like you, I have sometimes wondered why Mrs. Elton agreed to marry Mr. Elton. The same word that Tarn used comes to mind when I think of the possibilities: desperate. She reminds me of Mrs. Bennett in that sense. I got out my copy of Jane Austen and the Clergy and here are some quotes that might be helpful in general and perhaps in relation to Mrs. E's decision.

"The news of a clergyman bringing a wife into the neighborhood always caused a stir: it was taken for granted that she must be absorbed into the tiny social elite of the place. Everybody must call on her: 'visits in form', as Jane Austen called them in Emma, must be paid as soon as possible. ...After morning calls must come the formal invitations. A Mr Elton in his bachelor state could be asked to spend an evening playing backgammon with Mr Woodhouse; a Mr Elton married must for the first time in his ministry be asked to dinner at Hartfield. Emma who would have preferred to avoid him, knew that she had no option: she 'must do no less than others'."

"...but apart from the poorest of them [clergymen], who scarcely ranked above the emerging class of shopkeepers and clerks, they were not unworthy of consideration from a worldly point of view. A living of £300-400 a year, with a decent parsonage house offering free accommodation, placed a clergyman on a level with the lesser gentleman and made him far from despicable in the marriage stakes. Whatever his antecedents, his position as a freeholder and his connection with a landed patron qualified him as a gentleman."

As has already been mentioned, on top of this Mr E has some independent property and with the 10,000 she brings, they should do pretty well.

"The attractions might be regarded as considerable. Once the husband was beneficed, his wife obtained an independent establishment which included a house and garden and probably a farm. She could now entertain her relatives and friends: many a parson's wife became noted for her hospitality (Jane Austen considered it a black mark against Mrs Norris that her bedrooms at the parsonage were seldom filled). Marriage with a clergyman also gave a woman the opportunity to carry out social work, perhaps in conjunction with the lady of the manor, to whatever extent she chose. Parsons' wives were expected at the very least to distribute their husband's charity. In addition, the poor and needy might call at the parsonage house for anything from advice to money."

I'm sure this all appeals to a Miss Hawkins who probably sees herself getting to be queen bee and in the thick of things in Highbury society. Her sister has gotten married, and her friends and associates seem to be mostly married people so the handsome, popular, well-situated Mr. E must seem, to someone eager to walk down the aisle, to be the bees knees. Hope this added some additional information to consider.