You can probably get a full shop on Foma for price of one small apartment on Caly... I used to have several apartments, but have given up on estates in game for the moment since I'd rather waste funds on a few other hobbies in real life. Number of slots in Apartments in Tangerine on Rocktropia is a lot higher than other places in game, but finding those on auction is far and few... good luck.
Average listed in this thread sounds about right... around 2-5k for a G apartment and a bit less for others... If you can find several together it's nice but good luck with that. At one point in time I had about 3 shops on Foma all across the hall from one another, but it took a lot of wheeling and deeling to get there and foot traffic was too low to really justify it. I did make a little profit when I flipped them after I sold a few rare pets and items in there for a few k ped more than I bought them for, but all estates in game are a bit of a waiting game to find right place, right price, etc, unless you just plop funds down to pay for whatever auctioneer or shops have available right now. Several estate deeds are in various malls and shops. Price around. Most mall shops carrying apartment deeds are a bit higher than auction value, but in some smaller shops prices may be a bit lower than auction prices. Just takes some time to study the market and what's available. On Calypso every freaking building has almost the same design so it's easy to get confused if you invite guests over. Be careful and do visit the place you see a deed for to make sure it's what you want.
Apartments can be upgraded by 20 points for 2k, but only certain apartments can have that feature... ask Mindark before hand if the place you want is already maxed out on that option. The system they use to do that is all hand done by them and they glitch on it many times, upgrading without collecting fee sometimes, etc. Things may have changed since a few years ago, but I doubt it. Also... along with that when things change in big ways, like, oh say implementing a new game engine, you can expect most estates to stop functioning properly for at least 2-3 years before they fix it, then update again and stop them from working again in another vu.... system is extremely glitchy over the years. There's quite a history to it if you search forums here.