I'd probably put more value on a longer word like fucke.rs fuck.rs lose.rs win.rs winne.rs Then I'd be looking at restrictions for owning that extension and annual reg fees In general though 2 letters should be good.
The only 2 letter .rs domain I have seen sold was st.rs which sold for $280 a little over a year ago. They have high reg/renewal/trans fees when compared to others so that is something that buyer might take into consideration. It's also a country code extension for Serbia so that's about your best choice for target buyers. There hasn't been much demand for them or even sales for them so it might be a tough sell if you own/buy it. I looked at possibly buying some but the combination of high fees and low demand made me stay away from them.
You are very welcome. You can check Namebio for past sales history of domains if you ever want to see what previous domains sold for. Cheers
you can keep it for investment purposes since two digits are rare, and if you can get some readable ones, it will have a good selling price.