Short answer. A reasonable price. Article 1474 says that where the price cannot be determined at all the contract is inefficacious; but if the thing or any part of it has been delivered to and appropriated by the buyer, he must pay a reasonable price for it. Because you received and used the goods, you owe their reasonable value.
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.
Civil Code, Article 1474 — Indeterminable Price; Reasonable Price. Read the full provision →
What the law says
if the thing or any part thereof has been delivered to and appropriated by the buyer he must pay a reasonable price therefor
Civil Code, Article 1474 — Indeterminable Price; Reasonable Price. Read the full provision →
What the law says
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 →
You owe a reasonable price
Article 1474 gives the answer for your exact situation. It begins with the general rule — Where the price cannot be determined in accordance with the preceding articles, or in any other manner, the contract is inefficacious — but then carves out the case where the goods have changed hands. It says that if the thing or any part thereof has been delivered to and appropriated by the buyer he must pay a reasonable price therefor. Because you have received and appropriated the goods, you cannot keep them for nothing on the excuse that no price was agreed. The law requires you to pay their reasonable price.
Why delivery changes everything
Without delivery, a sale whose price cannot be fixed simply fails — it is inefficacious, and neither side can demand performance. The turning point is that the goods were delivered to you and appropriated — taken and used as your own. At that moment the law will not let a buyer enjoy the goods while hiding behind the missing price. It converts the arrangement into an obligation to pay reasonable value, so that a buyer who has already consumed or used what he received cannot walk away without paying. Having taken the benefit, you must pay for it.
What 'reasonable price' means
The article does not hand you a fixed figure. It states that What is a reasonable price is a question of fact dependent on the circumstances of each particular case. So the reasonable price is worked out from the actual circumstances — commonly the market or current value of such goods at the time and place, and the surrounding facts of the deal — rather than by a rigid formula. This gives flexibility but also uncertainty: if you and the seller disagree on the number, it becomes a factual question to be proved, weighing what these goods were fairly worth in your particular transaction.
The limits of the rule
This provision applies where the price genuinely cannot be determined by any of the usual means. If a price actually was fixed, or can be worked out from the parties' dealings or an agreed standard, the contract is not inefficacious and you owe that price, not merely a reasonable one. And the reasonable-price duty attaches only to what was delivered to and appropriated by you — you are not made to pay for goods you never received or accepted. Within those bounds, though, the message is plain: goods taken and used must be paid for at their reasonable value.