My friend and I have a deep affection for howdahs, so if you have one of them I'd say you're in business.
YEAH BABY! Are there stats for those in any D&D books? I've been dying to get a big zombie/skeletal elephant/rhino/whatever to use as a "mount" for my character in the Way of the Wizard game.
I'm glad it's not just us! I feel like there were, at some point, stats on a howdah -- the old (and often stupid) Arms and Equipment guide, maybe?
Off the top of my head, I would just say they give you cover, just like any other reasonable waist-high type barrier. I think that's probably the actual rule, but don't quote me on it. As to cost, I think it'd depend on the creature. For a rhino or elephant, creatures that are reasonably howdah-able in real life, I'd say that if you suggested 500 gp no DM could turn you down. For something wacky like a linnorm, offer a few thousand gp and, again, nobody can turn you down.
What's the short version of the "Way of the Wizard" game? It's got a great name.
Oh, and random old war story re: skeletal elephants:
[spoiler]
This is way back in 2E, where there was a spell Animate Dead Animal, which made crappier undead (they took a Str penalty, etc.) but did not have hit die limitations. There were also all sorts of spells letting you fuse bones together and the like. I think all this is from the enormous fun and flavorful Guide to Necromancers, blue book from back in the day. I took that and combined it w/ a couple of undead elephants and various skills and somehow managed to con my DM into letting me have a giant undead tank-type thing w/ a scorpion type theme, w/ the whole affair covered in plate mail.
That I managed to get that to fly is a testament to my D&D huckstering. Although, in all fairness to the both of us, I pitched the idea (and it was a pretty high-powered campaign) and he said something to the effect of "ok, that's real cool, how can we come up w/ stats that are reasonable" and we hammered it out over about 20 minutes and I, at least, was damn pleased. Of course, then there were the animated skull trapped fish that were "torpedoes" the birds that were "missiles" and, well ... you get the idea.[/spoiler]