Here's an idea for slayer, it's basically a complete rework but i'm sure that it works.
Let's start from scratch.
Slayer-wise, there are easy, medium, and hard monsters.
Killing an easy monster while doing your slayer task will award 1 point per kill. At the end of the task ( once you kill the last monster ) you will be awarded 50 points for completing the slayer task. For medium you will be awarded 2 points per kill, and 80 at the end. In hard you will be awarded 3 points per kill, and 100 at the end.
I'll let you arrange what monsters go into what category because I don't have a list of all the slayer monsters.
Now tasks that are given to kill harder monsters are supposed to be harder tasks, so increase the quantity of monster kills you need in the task. In order to be able to receive medium monsters, you have to have 50+ slayer. And in order to receive hard tasks, you must have 75+ slayer.
Elite monsters ( bosses ) can be given out as tasks, make it a rare task to get from the slayer master. Make sure that it's bosses only though. You can either have a low quantity of kills ( IE 5-10 ), or just 1.
If your going to go with the low quantity, make it 8-9 points per kill, and the completion award set to 70.
If your going to go with the single quantity of bosses, make it 10 points for the kill and 60 for completing.
To keep track of the points, you click the slayer skill and your guy will quick chat saying "I have X slayer points, and I must kill another x of (enter name here ).
Slayer shop :
Feel free to change the prices, this is what i'm comfy with.
Archer / seers / warrior rings = 300 slayer points
Berserker = 450 slayer points
(I) archer / seers / warrior = 600 slayer points
(I) Berserker = 750 slayer points
Dragon fire shield = 700 slayer points
Slayer helmet = 800 slayer points
Fire cape ( because some people hate Jad ) = 100 points
Saradomin Sword = 500 slayer points
Spirit shield = 150 slayer points
Blessed spirit shield = 800 slayer points
Spectral Spirit shield = 3500 slayer points
Arcane spirit shield = 4000 slayer points
Elysian spirit shield = 4500 slayer points
Divine spirit shield = 6000 slayer points
Amulet of Fury = 1500 slayer points
Staff of light = 700 slayer points
Arcane stream = 600 slayer points
Dark bow = 850 slayer points
I'd suggest removing crystal bow from the ranger shop, and putting it in the slayer shop for 850 slayer points.
zamorakian spear = 500 slayer points
Infinity Robe = 500 slayer points
Infinity Hat = 350 slayer points
Infinity Legs = 400 slayer
Infinity boots / gloves = 150 slayer points
Mages book = 200 slayer points
The prices I have listed are a good set, the spirit shields are exactly how they should be.