Timeframes are very relative. It comes down to strategy in how much time you will need to achieve something. Credits do just that as well; allow you to do things faster. So does experience.
Therefore, since Hideout is meant for experienced players, then the buildings there need to be expensive enough to challenge the veterans. The time that a newcomer will need to get there is not something that should define the feature. By the time a player decides to expand to a Hideout then he should no longer be regarded as a newcomer.
I will remind that game metrics constantly change and as developers, we need to ensure that our game economy is kept under control. Let us not forget that the challenge of getting gold is nowadays a lot easier than before. Getting 10M when this game started was like a huge chore that could take one player several months and still fail. Nowadays, this is not the case, you can get 10M in day 1 (through other players, always).
For example, we got a nice feature called Loans. Banks currently hold 16B at their vaults. Surely, a big chunk of this amount is available for loaning and anyone can take advantage of this. There are still 30B in circulation and many more in liquidate-able assets. So I would say that still, Hideout buildings are not as costly as you think.
But even if they are there is one more element to take in mind; our initial planning was that every player will get to choose either a bank or a goldsmith and this is why these two buildings are expensive. They cost about 2.2B-2.5B each (depending at large on how much you get to pay the gold bars). So, the true number is 3.5B and not 6B (If 6B is correct, not sure of the total cost). Now, if one player wants to beat our design and possess both, this is not something we need to facilitate (make it easier).
Finally, the time for an experienced player to get a Bank or a Goldsmith (lvl 20), has been calculated before hand and was a result of a case study we did. It has been metered at 6 months average. It was a good case study I can say now as it met real game metrics, at large. Some players will do less time and some will do more; this is what the game is about though; best strategy wins.
Btw, we have improved the calculator for Hideout buildings; some cosmetic improvements AND a total cost in gold bars AND gold coins at the bottom. You can either access it through your Mansion or if you do not have one yet but still want to see the costs you can visit it through this link:
http://s2.piratesglory.com/hideout_costs.php?var=bank