לזכות ידיד נפשי ר' שמואל צבי בן ר' דוד עקיבא לברכה והצלחה בכל מעשי ידיו!!
From the margin of the Holy Rogochover's gemara with a running commentary:
אך באמת למה לא נימא גבי פדיון הבן בעי שומא?
Why don't we say that Pidyon Haben requires a שומא [assessment of value] when giving שוה כסף?
עי' לקמן דף ח' דמשמע דאין צריך לשום.
On דף ח the gemara implies that שומא is not necessary. The gemara compares the need for שומא for Pidyon Haben to the need for שומא for Kiddushin. We pasken like Rabbah that שומא is not necessary for Kiddushin so it must not be necessary for Pidyon Haben.
ועי' ערכין כ"ז
Rav Chisda says in Maseches Erchin that שומא is with three people.
וסנהדרין ט"ו ומגילה כ"ג
In those two places it says that שומא is with 10 people.
We see there that שומא is with three people.
So we have a contradiction whether שומא requires 3 or 10. [See Rashi Erchin 27b who related to this question].
כריתות דף י"ג ע"ב גבי ערכין עיין שם.
There it says that when making a שומא one may not be drunk. So we see that שומא is a מעשה בית דין.
Rabbeinu [צ"פ הל' ערכין ח,ב] seems to answer that the actual שומא only requires 3 as a Beis Din but must be done in the presence of 10. He writes "השומא באמת די בג' אך בפני עשרה". He is alluding to that answer here when he demonstrates that a שומא is a מעשה בית.
למה מדמי הגמרא זה לקידושין
The gemara wanted to prove that Kiddushin requires שומא from the fact the Pidyon Haben requires שומא. But there is a fundamental difference between the two. Pidyon Haben is a redemption of kedusha [as we will see] which should require a שומא בבית דין unlike Kiddushin where this shouldn't be necessary.
דנהי דזה לא הוה פדיון
Many Acharonim maintain that the Bechor doesn't actually beling to the Kohen so we can't say that it is an acutal redemption.
מ"מ הוה הפקעה
Pidyon Haben removes the obligation of giving the 5 sla-im to the Kohen.
דשוב אין מוטל על הבן
Rabbeinu explains in many places that there is an obligation on the son to redeem himself but when the father gives the Kohen the 5 sla-im we view it as a דבר נמשך - a continuous giving of 5 sla-im to the Kohen which absolves the child of his obligation. So we learn that the giving of the money is a removal [הפקעה] of the obligation to hekdesh and not just a gift. This being the case, it is therefore different from Kiddushin which is just a handing over of money without any previous obligation. So how does the gemara try to derive Kiddushin from Pidyon?
ועיין מה שכתב ברא"ש בהלכות קטנות
The Rosh says that the father has to inform the Kohen that this is a bechor and the Kohen then asks if he would like his child back or would prefer to redeem him with 5 sla-im. So we see that there is a הפקעה going on here and it is not just a regular giving.
The Sfas Emes [Bechoros 9b] proves from this Rosh that the child really belongs to the Kohen and must be redeemed. Rabbeinu isn't necessarily going so far but he is saying that there is an element of הפקעת חוב here. See Chiddushei Rav Shimon Shkop [Kiddushin 16] and Shaarei Yosher [5/25] that there is a הפקעת קדושה. See also Minchas Chinuch [392/7] and Sforno [Shmos 13/2] and many other sources.
In conclusion, Rabbeinu is bothered by the comparison the gemara makes between the need for שומא in a case of Pidyon Haben and Kiddushin. Pidyon Haben is removing the obligation one has to hekdesh which should necessitate שומא unlike Kiddushin which is a regular giving and should not require שומא. The conclusion based on the fact that we pasken like Rabbah that Kiddushin [and thus Pidyon as well] doesn't require שומא is especially problematic because Pidyon should REQUIRE שומא.
[עפ"י צפנת פענח קידושין עמ' ב-ג]