Yep, moderately pricey. I actually already have a Topeak MTX TrunkBag EXP which actually has a larger nominal capacity, so I'm definitely leaning towards getting a Topeak seatpost mounted rack. I'm intrigued by the Silvio under seat storage idea. I'd also like some accessible during ride storage (for snacks and maybe a few other items).
1. handlebar:: not much space available with this handlebar, new cruiser-style handlebar may have more space
2. stem: already being used by smartphone mount
3. steerer extension tube: I'll probably make some water bottle holder permanent, but there's more space around the tube
4. telescoping front tube: I remember seeing a post which used a top tube bag here, but that's not really accessible during the ride.
5. top tube: not much space there, though I can move the seat pan back an inch or so
6. under seat:a side-entry clip or box/bag could be mounted to the bottom of the seat on either or both sides (basically a variant of the Silvio under seat storage idea)