That is a good observation. One of the faults I find with this series is that each medal has its own design and style, with no unifying theme or design element. It is because the designers were all different, each with their own style and interpretation, and the series is experimental. Hopefully the merits of the individual medals can override this issue.
Perhaps, I may venture to say that this apparent disparity in interpretation and design may paradoxically turn out to be one of the strengths of the medal series. There is always a sensitivity about "commissioned" design work because artistic independence could be impaired. If different artists have been given the licence to interpret and design these medals then there is likely to be vigor and heterogeneity in the finished work and I believe the medals so far attest to that fact.
Moreover, none of the imperial gardens represented so far by the medals are similar to each other, until you start breaking the medals into their constituent elements. That's when you begin to see the similarities, from the use of water features to containment areas, structural elements from bridges to rooflines, arches, pillars and porticos, and the interplay between straight lines and curves. Chinese gardens are multifaceted and capture features in the particular environment where they are situated. Therefore the medals should not look the same on cursory examination.
We have to go past the issue of cost and associated politics and focus more on the art work and technical expertise demonstrated by all parties involved in the design and production of these medals. Art historians can give a better account of what I am trying to say. I believe there is a lot of good in what these medals represent. Kudos to the sponsors, designers and producers!