I'm not really expert in the cost of magic items, but i can say it is not as cheap as 100gp.
First, the spell you are looking after is more of the kind of Faerie Fire or Unwelcome Halo, both lvl 1.
Second, to enable it at the same time of your Blood Reservoir you can use the same command word. The cost of such item is 1800 gp * caster level * spell level = 1800 * 1 * 1 gp = 1800 gp (see Magic Item Creation, under Magic Item Gold Piece Values > Table: Estimating Magic Item Gold Piece Values > Spell Effect > Command word: Spell level × caster level × 1,800 gp) very close to the Blood Reservoir. You just need caster lvl 1 because those spells last 1 minute/level. Suppose they modify the spell to works like a lvl 0 spell, then the spell multiplier for costs is 0.5, dropping the item cost to 900 gp; limit it to some rounds, maybe just 5.
Limiting the usages per day could drop the cost more:
- 1 per day = 900 gp / 5 = 180 gp
- 2 per day = 900 gp / 2.5 = 360 gp
- 3 per day = 900 gp / 1.67 = 540 gp
- 4 per day = 900 gp / 1.25 = 720 gp
- 5 per day = 900 gp / 1 = 900 gp
To say the item is just cosmetic is to underestimate the item; and, sorry to say it, a lie. It is powerful enough. You clearly stated you want it to be able to bluff foes. In addition: It can be detected by magic, so friends can find you using detect magic; anyway they can find you if you shed light where in darkness and some other concealing conditions, like fog ("Hey guys, just follow me!"); useful to send signals; it can make you momentarily visible when invisible without breaking the invisibility; it can ignore concealment (even provided by blur); among other creative uses.
Something the GM is not realizing is the character can use it to bluff a foe, making the foe believe it is that item what gives him the strength boost and not the Blood Reservoir. The monk is telling the orcs'leader "Boss, please spare the life of my companions, take this in exchange. It is a powerful item that brings you the strength of a bull, i'd teach you its word... You see? I'm bursting in power!". Then the party fight the orcs again in the middle of the night, the orcs' leader uses the item, no extra strength, the whole party clearly sees him and take it down with bows and some spells.