how to charge ?
1. What u wan ? Creating portfolio or try to make for living ?
2. What they wan ? Something u can't do by ur own or just simple clips.
3. Who are they ? MNC ? SME ?
4. Who are u ?
If they are MNC and wanted to give u a try without request any portfolio, i think those is China man cheap skate company, grab it, no worry (prices low but u earn back in portfolio) .....
If they are SME can charge them a bit higher but still within their budget.
From ur question, i can see u are fresh in business ..... so market prices is not what u need know ......
don't flame me if i wrong ....
quotation price for Corporate/Montage video