I'm not currently PG, but with both my previous PGs I gained in the region of 2.5 stone altogether, which I was happy with. The weight gain was pretty lumpy though, some weeks only gaining half a pound or so and other weeks gaining three or more. I read somewhere that in the last month of PG a baby is laying down up to half a pound per week in weight itself.
My BMI has historically been in the middle of the normal range (I've got lucky genetics, I eat all kinds of fatty cr@p, and seem to get away with it, the only thing that fattens me up is beer, which I cut down on years ago!). I think with the births I prob lost about half of the gained weight (baby, placenta, fluids etc), then by approx 6 months post birth I was back to normal weight, then both times with extended BFing I ended up being a bit underweight, and tired/knackered. Currently, my baby DS is a big fatty, and I'm getting a bit skeletal!
IME breastfeeding is a great way to get back in shape, I think, off the top of my head, that a baby drinks perhaps 750ml - 1 litre or so a day (can't tell exactly when BF!), which is prob 700 calories, all very roughly calculated.
I think with extended BFing I could have shifted 5 stone or more, now that I've done it twice I think I'd actually try to fatten up a bit in preparation when PG.