I was thinking that purchase of a larger home would result in achieving this one. When I originally bought my home, I only got the lower achievement, so I thought maybe not having achieved the lower achievement might've messed things up for me. Unfortunately, I'm not interested in the other homes enough to want to move yet, so I haven't gotten around to testing that thought. I do know that buying 3 (or more) crystalizers won't do it though. Perhaps because they're 3 (+) separate items. Or maybe it's a bug, or because there's no single item that expensive, or...?
I've made a purchase in auction of more than 5000 currants, and don't have the achievement, and by "item" I know they don't mean house, because that's the Homesteader award, so I'm guessing that it's a future achievement for when there will be an item to purchase that costs more than 5000 currants. Like, say, home decor...
this makes sense to me as some of the other spending achievements were not available until new items were added in game (toolboxes and El Big Spenderino)