Cracked on the toilet tank. No previous issues, just all of the sudden started to leak!
Any suggestion except replacing the whole toilet (was told $750) including labor :(?

Seal the crack?

You posted this twice.
Marine epoxy could work. Tank must be very dry. Some people also imbed a piece of vinyl pool liner in the epoxy over the crack. May work for a while, may not.

Sometimes ( but rarely ) - and if it is a regular, inexpensive toilet, you can purchase replacement tank. But most of the time, it is better to replace the entire toilet, including bowl.

Good toilets run between $200.00 and $300.00, inexpensive toilets are under $100.00. Prices for installation vary and you will have to shop around for best price. I don't see why you could not fit the entire project under $500.00 and less.

I do not recommend any glue-fix approaches as glued porcelain toilet tank will never be structurally sound and may cause injury should you lean against it and it breaks.

Hope that helps.

Milo