Stranger, considering how expensive these cages are and how small they are for that price - why don't you consider making a bin cage? It's much cheaper and will give your dwarf a lot more room that he'd really appreciate. They are not hard to make either.
Think about it, the mini duna is only slightly larger than a Makrolon T4 lab cage (which is 57cm x 37cm x 20cm = 2109cm²). Shouldn't a beloved pet live in something that's significantly larger than a lab cage? Now I know some people really don't have the space but you said you had the space only your mum was not so thrilled. Maybe if you told her about the lab cage sizes she'd allow you a bigger cage? Because I think it's great that you do your research before getting a hamster - many people just don't care. And I think it's a pity that some people made the mini duna look overly good. Take Katie's experience that her hamster developed a very bad stereotypy and that many people end up having to upgrade later anyway serious. It's usually cheaper to get a big cage in the first place than buying a small one and then upgrading later so you really could be wasting your money.
I don't know if you have an IKEA near you but if you could get the biggest 130liter "Samla" bin + lid + lid clips for only 15.80 pounds. Then you cut a big hole into the lid and glue on some wire mesh / hardware cloth with eg. hot glue. That'd be a much better cage for way less money. Or you can get a similar bin from other stores.
I've put up an album with my temporary(*) Samla setups
HERE how about showing those to your mum?
(*) I only used the Samlas as temp cages because where I live the minimum requirements are slightly bigger than even a Samla so I can't use them for permanent housing.