I am not sure what you want to indicate with "Ach ja" in the fist sentence. Ach ja works as a phrase realizing a fact in the sense of "Oh right!" which can have a connotation of regret and/or remembering something. Without the context it is hard to say if the usage is correct or not.
But to your first phrase: "Seit" is "since" in a temporal meaning. If you want to have "since" as a causal meaning, you need to use "da" or "weil". Then, there are 2 errors in word order. The correct phrase would be rather
Ach ja, da Sie vierundzwanzig Jahre alt sind, kostet er nur 36€.
Ach ja, Sie sind vierundzwanzig Jahre , da kostet er nur 36€.
Which is rather "Right, you are 24, then it only costs..."
Ach ja, er kostet nur 36€, da Sie vierundzwanzig Jahre alt sind.
Here I used the "er kostet nur" order, which requires it to be the main clause.
The second phrase is correct except you need a capital P for prize and 24 does not have capitals.
As to the difference between the sentences: The first one, as mentioned, with "Ach ja" indicates that the speaker just realized the described fact, the speaker might have been challenged on the prize by the customer and corrected himself.
The second sentence would be used when the speaker rather explains the prices to someone who inquires, and has the prize right away correctly.