I would count a huntable white rhino as likely the easiest of the big 5 to get. As for cost, if you take a deal on a dehorned white rhino bull, as advertised here, it would likely not be the most expensive, leopard can be done cheaper or more expensive, but you should expect 100% success on a white rhino. If anyone promised 100% on leopard, something is not legit or they are offering a return guarantee.
For importable lion or elephant, it will likely cost more. However you can find non importable to the US lion and elephant on this site for about half the cost of the dehorned white rhino deal. Importing a white rhino should not be a problem as long as all documents are done properly.
I would add, if you want the big 5, get the leopard and rhino asap. Then if willing to do non importable, get the elephant and lion. And if your willing to do non importable, getting the big 5 is really quite economical right now. With a bit of luck and taking a deal but with a good operator on leopard, might get it for $18,000? $25,000 on rhino. Can do non importable elephant and lion for $12,500 or less. And could do a tuskless and old lioness for a lot less. If you want an importable lion, i think it will be the most expensive. Then go for a great wild buff hunt.