Dropship Markup

Hello

I'm starting a dropship business, and i'm going to sell consumer electronics. I have contacted some wholesale suppliers who are willing to do dropship. But I need some guidance with price markup. I understand that margins on consumer electronics will be very small, and since i'm starting this for the first time, i understand i have to be very careful with this pricing strategy. I went through lots of calculation, like setting up a fixed markup based on cost price (ex: if cost price is $100, i can set a 10% mark up easily) and for some low price items, instead of mark up% i can use a fixed dollar amount (ex: if the item cost around $10, then i can increase it by $3.5 as a fixed amount), but the issue is... lets say i go with a combination of both the above approach...
Assume:
1. I have 100 products in my store
2. some items may be between $10 to $25, some between $25 to 45 and some more than 100... (all price ranges)
3. if my monthly expense is around $300
what should be the mark up for each item, also if i go with this approach, i'm sure not all 100 products are going to be sold every month... so can you help me figure out a way to make at least my monthly expenses...
Thanks
KPK