I will say that it is quite necessary to know how much progression you have so far, several factors impact the price, let me list some of these:
- Loyalty points (the more progression here the less gold spent on skills).
- Skills by themselves (there is a certain point where no matter how you progress 1 single skill is much more expensive than tier 1 or tier 2 items and they provide more or less the same impact), btw with exert weapons all skills (ml, dist, melee) cost the same.
- The type of Forge (None of the forged items have the same impact on hunting, so I will use only Onslaught as a measure as it is the most consistent and simple to explain).
- The price of the Forge/TC (Pretty obvious, but we must consider a base price for everything being measured in order to make appropriate data, in this case I will arbitrarily use TC at 35k, Sliver at 10k, Fusion Item cost 3kk).
Having said all that, here are some facts:
- From magic level 100 to 110 In double skill + private dummy you spend 128.52kk with 10% loyalty (5 days, 16h)
- The same magic level and conditions but with 15% loyalty costs 121k (5days, 8h)
- In a level 1000 character (using the Guildstats.eu calculator), every skill after 100 gives +2 damage, which is barely a 0.4% damage increase.
- To get these results (0.4% damage increase) on a char with 10% loyalty and 100 ml it costs only 7kk, with forge it costs 20kk using 3kk items (lets say a cheap wand).
- On the other hand, if you have the same character with 110+ ml, it will cost pretty much the same (22k).
- Now what about lvl? a lvl 1000 gets +1 damage base every 6 lvls, so it takes 12 lvls to get the same 0.4% damage increase.
With all these data we could say
in stages of 10 after ml 100, each 1 skill point costs the same as 1 tier giving 0.4% or less damage increase (it decreases the higher the skill).
This means that from 110 to 111 you spend the same in a t1 forge as in a training weapon (with cheap items), from 120 to 121 you spend the same in a t2 forge as in a training weapon and so on from 130 to 131, but:
- Forge progression doesn't escalate because you have to use fusion items to transfer them to your main weapon later.
- Forge skills don't degrade, in fact the higher the tier the higher the % increase, so a 140+ skill point might give +0.2% damage increase while a t4 forge item gives +1.47% (0.45% extra compared to t3) and they will probably cost similar.
- Skill progression is very different at higher levels of loyalty, as it is cheaper.
Conclusion
On average, you should either buy forged items based on your skill threshold (110/120, etc.), or just start using forging at 130+ skills, where it's simply more powerful than your skills in most cases, and just as expensive.
In your very specific case the threshold should be this:
6 lvls >= tier 1 items >= 1 skills > tier 2+ items
Where if you will buy the tier 1 items (and sell them later) they will impact equal or more than 1 skill and cost pretty much the same or less, but it's not worthy to forge items by yourself.
Disclaimer: Beside the Skill Progression is the same disregard the vocation, huge chances are that the skill impact isn't the same (specially for paladin and their diamond arrows) but since I haven't found a reliable calculator for those I won't dive too much into this, also, even if the skill impact is different there's a huge probability that skills impact less in both paladin and knights than mages.
Sources:
Tibia Wiki: Onslaught performance, Damage formula
Guild Stats: Damage Progression
InTibia: Skill Progression