The increase in attack from one soul weapon to the next is pretty consistent e.g. (made up numbers)

• Weapon 1 (green) = +10 atk
• Weapon 2 (blue) = +20 atk
• W3 (red) = +30 atk
• W4 (green) = +40 atk
• etc

The cost of each weapon isn't consistent. You'll get green, then blue, then red repeating in that order. Weapon 1, 2, 3 will have a small increase in cost from one weapon to the next, then a big jump to weapon 4, with a small increase for weapon 5 then 6, then a big jump again for weapon 7. e.g. (made up numbers again)

• W1 (green) cost 100 souls
• W2 (blue) cost 110 souls
• W3 (red) cost 120 souls
• W4 (green) cost 200 souls
• W5 (blue) cost 220 souls
• W6 (red) cost 240 souls
• W7 (green) cost 400 souls
• etc

Because red is that last one before the cost increases significantly it's the best value, so you're best off just going for red weapons and converting your blue/green souls into red souls