Assuming use of the Mondaha club with Realmrazer, you'd have something like:
1) 1/5 @100: base
2) 1/5 @200: +36% over tier 1
3) 5/5 @200: +8.7% over tier 2
As for whether it's "worth it", that depends on varying factors. You need to determine:
1) How often you use the jobs that would use the given weaponskill, to balance out where you're spending your merits. Likewise, how many spare merits you have to spend, depending on how many of the weaponskills you want.
2) To what degree you need to do damage with this job (ie: main DD, or DD mage playing around).
3) The base damage of the weapon you're using. The higher the base weapon damage, the less valuable additional merit tiers are.
4) How it compares to non-merit weaponskills; with some of the new delve weapons, non-merit weaponskills can catch up to or suprass merit weaponskills. That's another discussion, though.
5) What percentage of the job's damage comes from weaponskills. The more damage that's weaponskill-based, the more valuable higher merit tiers are. A job that's 70% melee/30% weaponskill will gain far less in real, total DPS than a job that's 30% melee/70% weaponskill.
Given all that, you'll largely have to decide for yourself how much is worth meriting for any given weaponskill.