This Chinese idiom is not suitable for the case u mentioned. This idiom comes from a very old and famous Chinese book ' The Mencius' originally . The literal meaning of it is : You gotta get the fish in the river and u gotta get the bear's paw in the forest coz you hunt the bear in that place, but you can't do these two activities at one time so u can not get the fish and bear's paw at the same time. Basically it tells people that you should make an alternative decision when you've got two choices which might have conflict with each other due to certain reason. EXAMPLE 1: A : 每个东西都很贵, 为什么？ B: 因为每个东西都是上品. 你想要买他们么？ A: 恐怕不能，我没有这么多钱. 鱼与熊掌不可兼得呀～ （coz u don't have enough money） EXAMPLE 2: A：下周我们有三天假期哦。 We will be having a vocation of three days next week. B: 是的。我们找个地方玩吧。Yes, let's find some places for spending the vocation A：我想去欧洲跟美国。 I want to go to Europe and America~ B： 时间不够，你只能去一个地方。 No, the vocation is too short, you can only go to one place. A： 哎呀，鱼与熊掌不可兼得。 Oh~ 鱼与熊掌不可兼得.

The example you presented here is not completely correct. When we use"鱼和熊掌不可兼得", we usually mean two things that we desire badly and we can also GET（得here means to get something） both of them easily, in a way, it's usually hard to make a decision between them so you have to choose either of them. According to that, it 's not related to the price or value of the subjects. For example, “Mary遇到了一个难题，如果她既想得到全职工作的实习证明，又不想放弃现在的学业，但是‘鱼和熊掌不可兼得’，因此她只能从中选择一个。” Hope you can understand it~~ Good luck!!

产品的质量好，价格当然贵，所以鱼和熊掌不可兼得。means you cant get two at the same time, example it's so hard to get one high quality and cheap. hope to help you

this idiom means 两个不相同的事物or 两个不相同的目的 不能同时达到。so i think your sentence doesn‘t make sense，婚姻（marriage）和自由（freedom）是鱼和熊掌不可兼得的，because marriage would limit your freedom。when you got one， you would have to lose the other one.

The closest English equivalent I can think of is "You can't have your cake and eat it too."