The gap in cost between level 5 and 6 is just too big for the edge it gives to justify the edge it brings. The pay2win gap in Splinterlands however is quite big with so many different cards available with each of them such a wide range of power levels.

Level 5 so far has been providing a good value/result ratio. Same goes with Level 3 for my alt account which focusses on Gold cards more.

Good Luck!