When you need a translator to accompany you to factories, what rate do you think is reasonable for an English-Chinese translator in Mainland China?
The question depends entirely on quality. I live in Mainland China, and I can tell you that professional native-English speaker copy editors generally charge above RMB150/hr when editing previously translated written material.
USD120/day converts to RMB 775. Depending on the level of translation quality you want, as well as your travelling needs, you can certainly find some one in that range. Remember that young people in China are accustomed to working long hours, so if you need a 12 hour workday, that's perfectly normal.
You'll find that some recent university grads with degrees in English won't be great communicators in English, but can represent your ideas well to colleagues and counterparties in Chinese. Chinese-based English education tends to stress listening and reading/writing, as opposed to oral communication. If you are looking for some one who will be translating complex ideas back to you in English, you might want to employ a more stringent vetting process.
If you post the location/region of China where you will be travelling, I can offer some suggestions on different freelancer websites in China where you can post an ad. I would also mention that depending on the region where your production facilities are located, you might do well to find some one who speaks the local dialect. For example, if you are travelling in the Pearl River Delta region (ie, Guangdong Provence), you will find that Cantonese is the lingua franca, but many factory managers are from elsewhere so you will want a translator who speaks Mandarin as well. If your facilities are in Sichuan, you will want a Szechuanese speaker (unless the factory is in a major city, where Mandarin speakers are common).
Post some more info, and I'd be happy to help you out as I can!
To me, it would depend entirely on the quality of the service and what I'm using it for.
I wouldn't pay nearly that for most personal uses, or for the quality of English-Mandarin translators I'm used to seeing.
For the kind of near-flawless translation I'd need for business purposes, I'd happily pay that much or more.