if it is only very occasional use, consider renting.
Or, buy one of the low-cost lenses used or refurbished (for crop I'd recommend the EF-S 55-250), and buy a lens more suited to what you shoot most (fast prime, UWA, ...). That way you don't loose money if you decide to upgrade later to a better tele. It also gives you more time to figure out what you really want next