a friend put a cat flap thro her extension, in the brick part, the cat was very happy wit it.
I'd second getting two, one will be lonely by itself.
Costs for cats, food, we give ours wet pouches twice a day, Felix as good as it looks, and buy massive variety boxes (about £12 on offer, we buy two a month), also have dry food on hand (its good for their dental health), and treats and tuna and sometimes cat milk because they love it so give it to them occasionally as treat, there's the kitty litter, ours are fussy and will not go in the tray if there's a litter they do not like! We flea treatment ours every three months, and cat toys, and I've got ours little covered cat houses like little igloos, they like to go in them and sleep but not necessary.
Basically when we go out shopping we make a point of bringing them something as well, they go thro the bags (yes really), the older cat likes the noise plastic bags make!
ours have full roaming over the house and prefer (my side) of the bed right when I am wanting to get in and sleep!
Ours are indoor cats and like sitting in the window meowing at passers by, (DP calls the younger one , also the loudest, ASBO cat, he's convinced he's sitting in the windows shouting cat obscenities!!).