Short answer. If the price genuinely cannot be determined, Article 1474 of the Civil Code calls the contract inefficacious, not a valid sale. But if the buyer already received and kept the goods, the law still requires payment of a reasonable price — a question of fact decided on the circumstances of the case.
What the law says
Where the price cannot be determined in accordance with the preceding articles, or in any other manner, the contract is inefficacious. However, if the thing or any part thereof has been delivered to and appropriated by the buyer he must pay a reasonable price therefor. What is a reasonable price is a question of fact dependent on the circumstances of each particular case.
Civil Code, Article 1474 — Indeterminable Price; Reasonable Price. Read the full provision →
No determinable price means no effective contract
Article 1474 addresses what happens where the price cannot be determined in accordance with the preceding articles, or in any other manner. Its answer is that the contract is inefficacious — it does not produce the effects of a sale. The article assumes you have already tried and failed to fix the price under the other rules the Code provides for determining price, and that no other method is available either. Only once every avenue for determining the price is exhausted does this consequence follow.
The exception once goods change hands
The article does not stop at declaring the contract ineffective. It adds: if the thing or any part thereof has been delivered to and appropriated by the buyer he must pay a reasonable price therefor. So a buyer who has already taken the goods and kept them for their own use cannot rely on the missing price to walk away without paying anything. The obligation to pay something survives even though the sale itself could not take legal effect for lack of an agreed price.
What counts as a reasonable price
Article 1474 does not fix a formula for the reasonable price a buyer must pay in that situation. It says only that what is a reasonable price is a question of fact dependent on the circumstances of each particular case. That means there is no shortcut answer here — the figure depends on the specific goods, the market for them, and whatever else is relevant to what they were actually worth at the time, rather than on any number written into the statute itself.
What this means before you sign anything
Because an undetermined price can leave a transaction without full legal effect, the practical lesson is to fix the price, or a clear method for fixing it, before goods or money change hands. If you are the buyer and have already received and used goods where the price was never pinned down, expect that you may still owe a reasonable price for them even though there was no binding sale in the technical sense — the absence of agreement on price protects neither side from that residual obligation.