The concept of "value" is a rather dodgy one. If you buy a car with X money from a shop, its value drops dramatically after the first minute when you drive it out of the shop as it becomes a used car.
I would argue it's impossible to determine the value of a token. But the price of a token is a bit easier though it may go wrong as well.
If you send a transaction with 10 Ethers to a contract and you get X tokens back you might claim that you now know the price of the tokens. Well, what if the crowdsale contract for example gives you the same amount of tokens after a month? Or returns some of your Ethers after a while? It may be impossible to know what the real price is.
But, in typical scenarios you can make an educated guess that if you send X Ethers and get Y tokens back the actual price at that moment is known. This is probably the case in at least 95% of crowdsales.