The gemara says that the minimum price that one must spend on an asham, even an asham taluy that is brought when one may have violated an issur for which they would have to bring a chatas (if they were sure they violated it), is 2 selah. The gemara suggests that we need a special limud for this because logic would dictate that if a korban chatas which is for a definite violation has no minimum that must be spent (although the gemara says a danka, rashi and tosafos both explain that it is an exaggerated small amount, but not literal), certainly an asham which merely suspends punishment until he realizes that he did the aveira and brings a chatas, should not have a minimum amount. This logic seems pretty solid, yet the halacha is that an asham taluy has a minimum amount and a korban chatas doesn't (Tosafos points out that as a mitzvah min hamuvchar it should be at least one selah - half the value of an asham). Why?
Rabbeinu Yona (brachos on rif 2b) explains that when one commits a definite aveira he focuses on his sin, worries about it, regrets it and does teshuva. But, when one violates a safeik aveira he justifies his actions by saying that he probably didn't violate anything at all. He rationalizes by saying that he probably ate the kosher piece of meat rather than the treif one and doesn't focus on teshuva. Therefore, the Torah demands that he spends more money on his korban. Rabbeinu Yona doesn't explain how spending more on the korban will make up for the lack of teshuva. Perhaps the idea is that by having to spend more on the korban he will realize the severity of the prohibition, and that will inspire him to do teshuva. Or perhaps the extra money he is required to spend will be a kappara even without the same level of regret.