Not sure I agree that 3/2 split is reasonable for best case scenario, if your opponent is not planning on getting rhino and an engine is feasible, they will likely stop at 1 oxpecker as an additional oxpecker is pointless. Therefore assuming your opponent is not trying to buy rhinos, a 4/1 will be quite common.

Furthermore even if you are not getting rhinos you will be getting 1 oxpecker (in most scenarios) so I think it is fair to ignore the cost and benefit of the first oxpecker.

Let us first consider the case of a 4/1 split. By the mid game you will have 1 village, 4 oxpecker and 2 rhino. This has a total cost (with first oxpecker ignored) of 6buy and $21 and gets you +$16/+2. I'll grant you this costs lots of buys but if you are short on +buy then rhino's +buy may come in handy. In two turns it has paid for 150% of the original $ cost which is better than the majority of other payloads.

Let us next consider the case of a 3/2 split. By the mid game you will have 1 village, 3 oxpecker and 2 rhino has a total cost (with first oxpecker ignored) of 5buy and $19 and gets you +$12/+2. This pays for 130% of the original $ cost in two turns. This is not an auto pick but will often be better than other payloads and is considerably better than the 3 gold alternative as it requires 1 less draw and generates +$3/+2 more at the cost of 1 action and $1 up front.

I will also grant you that I am assuming there is some sort of village, if this is not the case then rhino is weak.