We don’t have an exact amount, no, but we can presume it’s pretty healthy, for a clergyman. It’s even remarked upon that Mr. Collins has been very lucky to have early prosperity, as a man in his mid-twenties with no particular talents. While the parish at Hunsford is a rural one, I feel like it must be more than even Austen’s own father had as rector of Steveton, where he earned about 100 pounds per annum from the living. Richer parishes could offer up to a thousand p.a., and considering the tithes that must come from the Rosings estate alone, it’s no wonder that the Hunsford parsonage is considered by everybody to be a worthy and comfortable property, even if the house itself is not large. Lady Catherine very likely tosses off all sorts of gifts (particularly game and fish and produce from her estate,) to her favourite clergyman, which is cheap and easy for her to do, and keeps him sycophantic and well-fed. Likewise, the invitations to dine more lavishly at Rosings Park are next to no effort for her, but she must be aware of the effect and impression they have on Mr. Collins. For Charlotte, this preferential treatment alone must be a boon to her grocery budget, as meat and rich dishes don’t come cheap. There’s also the expectation of the inheritance of the Longbourn estate at 2000 a year, which could easily triple even the income from the richest living (1000 p.a., as I mentioned,) and Mr. Bennet could go at any time and isn’t going to have a son…so it’s not only Collins’ in-hand capital and income and favours from Lady Catherine that give him quite a comfortable life by any standards, but the expectation that in likely a few years he will come into a gentleman’s property and not even have to work for his living. (He could then install a curate at Hunsford to actually do the services and still take the lion’s share of the living’s income for himself while he lives at Longbourn.)

I’d have no trouble believing Mr. Collins’ Hunsford income to be above 500 pounds a year, at least, and very likely closer to 1000. (Bear in mind that the sensible young newlywed Ferrars at the end of Sense and Sensibility are living on an income totaling just under 1000 p.a., and would not have thought it possible to marry on less than 500 a year, and Mr. Collins might still have other streams of income from other properties he has inherited or investments/banking that is earning him interest on lump sums. Also, the Delaford living is probably still a poorer parish than that belonging to a grand estate like Rosings.)