I currently have a Thermarest Scout which is a good compromise between price and comfort. I bought it for $60 CAD. Good insulation (R 3.4) and packs down to about 5.5X11" and about 1lb's. I put it between my aerobars but It will fit in a Salsa anything type fork cage or similar space. Full length mummy shape.

I spent a bit of time comparing it in store to other inflatables and many did not have that big a savings in size or weight until spending at least $150-200. Maybe one day but at the time that was too rich for me.