You can start as little 1 or 2 steem and then you can try using the bot to see hot it work. Look at bot minimum bit column and accordingly you can bid the minimum the need. Some ask for 0.1 sbd and some ask 5 sbd.
1 steem is about $1.7. Start with 500 rs and from there you can decide if you want to invest more.
Le me know if you have any questions