Digiscoping with an SLR offers a lot of challenges, even more so on a budget.
I have had luck with the Vortex Razor HD, the Canon 50mm prime, and affixing the setup with Vortex' dedicated Digiscoping adapter. You will be limited to manual focus however, and you won't be able to use focus confirmation.
Better setups can be had with Swarovski's scopes and their UCA adapter, or you can turn most high-end scopes into a manual focus lens via a t-ring and a dedicated SLR adapter.
Swarovski's new TLS-APO looks to bee the bee's knees for SLR work, but there isn't anything inexpensive about it.
