Rashi and most of the Rishonim understand this to mean that once the בעלים express an interest in being פודה we force them to pay extra קרן if someone else bids more even if they don't want to. It comes out that even thought the בעלים only may have wanted to pay 25 we force them to pay up to 30.
The Rambam has a completely different פשט in this Mishna in ערכין. The Rambam (ערכין ח:ה) writes that if the בעלים say 20 and someone else says 21 and the בעלים say nothing (they make no counteroffer), the בעלים get it for 25 (קרן + חומש). The mishna in ערכין is talking about where the בעלים make a counter offer of 21 and a פרוטה. Then they have to pay 26 + a פרוטה. As the Raavad there points out this is very difficult in the לשון of the Mishna and the sugya there. According to the Rambam why would the בעלים make a counteroffer when they can just stand by their original offer and get it for 25?