I guess you are going to have to define affordable for us because brass is pretty expensive. I would go with a 5" diameter and 8.75" length for a Coehorn style mortar. This would be approximately 53 lbs of brass which would cost around $200, maybe more, just for the barrel metal. The trunnion would run another 4 to 5 lbs. Then add something for labor, something for a crate to ship it in, something for shipping.
Going to be hard to beat Wild's $550 price, just for a barrel. Besides, their mortar mount looks like it is backwards and will tip over with the first shot unless the barrel is pointed the opposite direction.