Fusing a monster with the same attribute gives that monster 1.5x more EXP.
In the picture shown above, if you feed a Darkness attribute monster to a Light attribute monster your base monster will only get 2250 EXP.

But if you feed a dark attribute monster to another dark attribute monster it will give that monster 3375 exp.

One Glumber at Lv. 1 will give 675 EXP.

Using two Glumbers as EXP material will net you 1350 EXP (675 x 2) and it will cost 3200 gold. However, if you use a special technique it will increase the amount of EXP and decrease the amount of gold needed for fusions.

By fusing the two low tier monsters together and making them max level, it will increase the amount of exp and decrease the gold needed for the fusion. Shown here, the Gold decreased to 3100 and the EXP shot up to 3375.

In the photo above, it costs 3100 gold to fuse a Glumber to a Lv.1 monster. However, the higher level your base monster is the more gold it will cost to fuse monsters into it.

The picture above shows that it costs 3500 gold for fusion to the Lv.5 monster
So it’s good thing to remember that higher level monsters need more gold for fusions.