The proposal of Mr. Collins to Elizabeth comes in Chapter 19 of the novel and is a typical example of his stupidity and lack of imagination. He explains it is his religious duty as a vicar to marry and above all that Lady Catherine de Burgh has recommended that he do so. A marriage to Elizabeth would help heal the breach between their families and also provide for Elizabeth's mother and sisters in the event of her father's death. Generously (note the irony here) he is willing to ignore Elizabeth's lack of money that she can bring to a marriage. Despite Elizabeth's refusals, Mr. Collins insists on believing that this is an example of her modesty or flirtatiousness, in keeping with the behaviour of "elegant females".
Humour abounds in this proposal. The speech of Mr. Collins is stilted, pompous and focused on an overbearing egotism. His wordy method of discourse leads him to structure his proposal as if it were a business plan ("firstly... secondly... thirdly...") which is hardly suitable for a declaration of love. Throughout his proposal he is so sure of his many merits and virtues that he has not considered Elizabeth's feelings or the possibility that she might refuse him. There is a note of tremendous irony when he says: "But before I am run away with my feelings on this subject..." There is no character more unlikely to run away with their feelings in the novel! His reference to Elizabeth's low expectations and financial situation is indelicate at best and is perfectly in keeping with his arrogance and belief in his own self worth.
However, often with Austen, humour masks or accompanies serious social realities, and these are again unsympathetically made very clear by Mr. Collins:
...and you should take it into farther consideration that in spite of your manifold attractions, it is by no means certain that another offer of marriage may ever be made to you.
Thus the economic realities of wedlock appear in their most unattractive light, and Lizzie is forced to confront the reality that by denying Mr. Collins and holding out for a marriage of love, she may never marry at all. This foreshadows Charlotte's acceptance of Mr. Collins for security and a position.