The first step is to figure out the cost of the attack itself. Two linked powers will simply cost the amount of all powers involved, which must be of the same range. The damage part is simple. Damage is 1 pp/rank. Adding the Line Extra adds 1 pp/rank. Multiple 2 pp/rank by 4 and you get 8 pp. The Affliction is fairly simple too. It starts a 1 pp/rank. Add a -2/rank Flaw for Limited Degree, add a +1/rank Extra for the line area, and then the -1/rank Flaw for the Limited to a single fixed target in the area (I'll comment on this part later). That gives a total cost of (1 - 2 + 1 - 1), which would seem to be -1 pp / rank. Fortunately, that's covered under Fractional Costs. 0 pp / rank is treated as 1 rank / 2 pp and -1 pp / rank is 1 rank / 3 pp. So four ranks of this power will cost you 2 pp due to rounding up (5 or 6 ranks would also cost 2 pp).
So right now, it costs 10 pp for our linked powers. Now, we apply the Removable modifier. Multiple the total cost of the power by 2/5 (-2 per 5 pp), which gives us 4 pp (you always round up this number). That gives us our discount. Our boomerang will be 10 - 4 = 6 pp.
Alright, now we run into other details. Your Limited on your Affliction to it always only affecting the last target is mildly shady in that it eliminates the cost of the Area it's applying to, making it the same cost as a Ranged power. It can't miss, but a successful Dodge check will halve the ranks of the effect. If you're affecting a single target, I'd say that's a fair enough tradeoff. Other GMs might disagree, so I'm just pointing out that it's not entirely in the rules.
Lastly, you mentioned that the character is PL 10. Your power is only rank 4. Area attacks lack an attack roll, so they are limited to your Power Level, which means that your attack is very underpowered. An average target with no tradeoffs will halve the damage 85% of the time (roll 1d20 + 10 to match the Dodge DC of 14). With full damage, they'd roll 1d20+10 as a Toughness save versus DC 19, which means no damage 60% of the time. If they saved for half damage, they avoid injury 70% of the time. I believe that comes out to about a 68.5% chance of no damage.
Things are even worse with the affliction, where 85% of the time, they're only facing two ranks and it's a 95% chance of avoiding any effect, 85% if they're facing full ranks. That's a 93.5% chance of having no effect from the Affliction.
There are, of course, no rules saying that you have to have attacks at PL, and many GMs will even limit ranks for Linked or Perception-range powers, but this is a bit extreme.
