Let's not get carried away; isn't a "gold standard" LSI controller only ~$1,000 ($27/drive)? But you certainly can buy a lot of Intel cores for that price.
The advantage of the hardware raid card is the battery backed cache. If it doesn't have a BBU and a fair amount of cache, as far as I am concerned, you might as well be using MD.
Hardware RAID cards have improved quite a lot recently; some of the stuff now has reasonably sized caches, so perhaps I should revisit my assumptions in this area. Of course, I'm planning on using ZFS on my storage servers, so even if hardware raid cards are now a reasonably good deal, they won't do me a whole lot of good.
But, I think I understand what you are on about now.
Most of us (well, speaking for myself, but I think this is true of most SysAdmins) have very strong experience telling us "more read cache is better" - I mean, more read cache, up until you can cache everything the server commonly reads, makes an absolutely huge difference in performance.
So we look for big caches.
The problem is that most of us don't have the same intuitive grasp of where the benefits stop coming when adding more space to the write cache, as most of us don't have a whole lot of experience with large write-cache systems (outside of netapp/emc type boxes, and I personally attribute their superior performance in part to their gigabytes of ram that can be safely used as write-back cache.)
So if write cache works the same way as read cache? yes I will pay the premium for the fastest 32GiB SSD I can find, if I can use it all as write cache.
The thing is, I'm told, that after a few gigabytes, the returns to adding more write cache fall off sharply; and if that's true, then yeah, you are right, 'cause you are wasting most of the SSD.
I mean, the real question here is "how much write-cache do I need before I stop seeing significant benefit to adding more write-cache?" and if that number is much above what you can get in a RAID card, then the zfs/ssd setup starts looking pretty good.
I buy most of my supermicro stuff through kingstarusa.com - I know the site looks a little shady, and you have to email for quotes for almost everything, but they are good people. My office is actually above their warehouse; I'm unit C. Their price is usually a few dollars less than provantage, which is usually the next best retailer for supermicro chassis, and they don't do shady tax dodge bullshit, and I don't have to pay shipping. I could be misremembering on the exact price on the 45 bay.
Most of the 'mapping out' done with the backblaze version is already done and tested on the supermicro.