In Dominion my family sometimes complains, that the game is over to fast because they couldn't build up their deck. While Dominion is designed to work that way, it might be useful to try some rule modification to slow it down somewhat while people still can buy the different action-cards. Have you a way to this that you recommend? How for instance would making money and point-cards more expensive would work out?
Things that require no actual house rules:
1. Get and use the Prosperity expansion (or the Dominion Base Cards set), and play more games with Colonies/Platinum.
Games with Colonies usually last longer, because it is not often worth buying Provinces. Instead of trying to get to $8 for Provinces, you are trying to get to $11 for Colonies.
2. Use setups that create longer games.
Some setups make the game go longer than others. Games with attacks will last longer on average than games without attacks. Games with more powerful cards like Wharf or cards that gain more cards like Workshop will be shorter on average.
Cards like Philosopher's Stone (from Alchemy), and Counting House (from Prosperity) become better in longer games. Any cards that give out Curses usually make the game go longer, because people's decks will be clogged. Strong trashing cards like Chapel will usually make the game go much faster.
Finally, alternate VP cards like Vineyards (from Alchemy), Duke (from Intrigue), Gardens (from Dominion), and Silk Road (from Hinterlands) can make the game go much longer, because one player can ignore Provinces completely and go for points with those cards instead (if the rest of the setup makes that a viable strategy). It takes 1 person a very long time to drain the entire Province pile, so if (in a 2 player game) 1 player is avoiding Provinces completely, he can make the game go long enough that his Vineyards and Dukes will be worth more than the Provinces that the other play managed to get.
I have not personally tried either of the following (as I prefer to play multiple faster games than fewer slower games), but a couple ideas that come to mind:
3. Increase the number of Provinces available.
If playing 2 player, you can use all 12 instead of 8. For 3 or 4 player, you could use 15 or 16. Either take the extra cards from the Intrigue set if you have it; or just use blanks or unused cards as proxies. The biggest issue here is that players will need to learn to not start buying Provinces as early as normal. If they start buying Provinces as soon as they hit $8, then the first 8 Provinces may be bought up, but then it may become unlikely to ever be able to buy the last few because your decks are clogged. This will also not make any difference for some setups.
4. Increase the cost of Provinces, like you suggested.
You do have to be careful with this, because in certain setups it can be literally impossible to ever reach certain amounts on a given turn; like if there's no way to increase your handsize. This will also not make any difference for some setups.
Remove Gold from the game. If you want to go further, remove Silver from the game too. This will limit which Action cards can be used, as some of them specifically work with the money cards.
When I first started to play, I didn't buy much Silver or Gold. Then I played a friend who basically only bought Silver and Gold, and not many action cards. He demolished me by being able to afford Provinces a lot sooner than me.
4 Ideas we Use:
Make 2 or 3 Stack of Pointcards and Mix the Pointcards, so every Stack. is random. So you are forced to wait or buy bad cards And more buying Options can be very valuable. Play it until one (or two) Stacks are empty. Use the Normal Number of Pointcards.
Use more Cards :D
Use Aggressive Cards which make people throw cards away or polute there decks with useless cards
A rule until you are not allowed to buy provinces. (One Stack of the Actioncards has to be empty, the Provinces are under some Mansion Cards(don't know the English Word), or a random Pointcard Staple (like in Suggest 1) has to be Empty) to buy Provinces. Be creative ;-).