You don't have to pay upfront, IBM in fact prefers you don't
Leasing, or frank out renting aka "cloud" that just happens to be IBM is the preferred form, especially as they can sell you on usage based pricing (good if your workload follows common pattern of spiking at end of month)
But profitable to IBM, and counts as OpEx not CapEx for accounting. A bit like cloud. But if you want they will ship it to you, or just setup a VPN or even a more dedicated connection (say, MPLS) to one of their datacenters. Or even sell it to you cloud style, running on LPAR/zVM.
They also tend to send you a more filled out mainframe (more CPU, more memory) so you can be flexible with utilization or "pay on demand" for more occassionally.
Leasing, or frank out renting aka "cloud" that just happens to be IBM is the preferred form, especially as they can sell you on usage based pricing (good if your workload follows common pattern of spiking at end of month)