For 5-20 MVA you're talking about 67 kV substations or similar, where a transformer costs in the low 6 figures. HVDC converter stations in the same range would cost somewhere around 8 figures, although that's mostly a guess- you typically need maintenance and supervision in a way that you don't with transformers. 1%+ downtime is pretty common, which absolutely sucks if you aren't a full grid and can't pull extra generation.
Yeah, but that price tag is mostly due to the fact that HVDC isn't a widespread technology yet. Once factors of scale come into play, the situation will look different and the prices come down.
Additionally, the price of copper is already at an/near the all-time high and it's not going to get cheaper, and the land on which huge transformers sit is shooting up in value... so in the end, market forces may push towards solid-state technology anyway.