The lvl 1 Mana Arrow has a Magic attackpower of 230 (not including the extra dmg from your MA stat). In comparison the lvl 10 Mana Arrow has an Attackpower of 275 which is only 45 more dmg per cast.
The difference why i think lvl 1 mana arrow is better in the beginning is that mana arrow lvl 1 only costs u 50mp,has same casting time and only a cooldown of 1sec while in comparison to a lvl 10 Mana arrow the mp cost is 120 (more than double mp cost...) and the cooldown is 1,9 sec (thats nearly double the time u have to wait to cast another arrow).
So all in all u do faster dmg with the lvl 1 arrow and it costs u way less mp which also saves u repotting time.
I hope this little explaining clears your doubt about only having a lvl 1 arrow in the beginning cause i doubt u have enough nice ma equips to onehit kill anyway even if u use lvl 10 arrow.