Shmuel holds that you can figure 1/6 based on מעות as well, and therefore if the price is 5 and you pay 6 that is אונאה of 1/6.
The Rishonim discuss the following case. The price is 51 and he paid 60. Based on the מקח he has overpaid by 9/51 which is greater then 1/6, however based on the מעות he only overpaid 9/60 which is less then 1/6. The same applies vice versa. If the price is 60 and he pays 51 based on the מקח he underpaid 9/60, less then 1/6, however based on the מעות he underpaid 9/51, which is greater then 1/6. What is the din in these cases? The Rambam paskens that you always go with the מקח and therefore where the price is 60 and he paid 51 the מקח is קיים. The מגיד משנה there asks on the Rambam how could he say that? We pasken like Shmuel that there is אונאה based on מעות also and therefore since based on מעות he underpaid more then 1/6 the מקח should be חוזר. The Ritva brings the שר מקוצי who agrees with the מגיד משנה while the Ritva himself agrees with the Rambam. Tosafos ד"ה שתות also seems to agree with the Rambam.
The ערוך השולחן explains the Rambam (and really the Ritva seems to say this) as follows. If based on the מעות it is 1/6 (e.g the price was 5 and you paid 6) then we can say that this is 1/6 and the מקח is קיים and the one who was cheated gets his money back. Why? Because no one loses. However, in a case where based on the מעות it is more then 1/6 and based on the מקח it is less then 1/6 (or vice versa) we have a contradiction. Based on one the מקח should be cancelled and based on the other the מקח should be קיים. Since we have a contradiction we need to pick 1 צד and since the עיקר according to everyone is price, we go with the price.
It comes out that we have the following anomaly. If the price is 50 and I paid 60 then the מקח is קיים and I just get the extra 10 that I paid back because it is 1/6 based on the מעות. However, if the price is 51 and I paid 60 overpaying by less (only 9) the מקח is completely בטל because based on the מקח I overpaid more then 1/6.
To summarize, the Rambam וסייעתו hold that where it is not exactly 1/6 then you always go with the מקח and Shmuel only said his דין of מעות where it is exactly 1/6. According to the מגיד משנה and others we go with both unless it is exactly 1/6 based on either the מקח or the מעות then the מקח is קיים and the extra money is returned. However, if it is more then 1/6 either based on מקח or based on מעות the מקח is חוזר. Only if based on both מקח and מעות, it is less then 1/6 do we say the מקח is קיים.