I've arrived to Hong Kong today and asked in a travel agency (CTS) about getting a visa to China. I'm Spanish and I got a previous visa. They have asked for my passport and after a couple of minutes they asked for my old passport where I had a previous (2010) visa to China. I told them that I don't have it with me and then they asked me for 3500 HKD in order to get it without the old passport. Is it normal? Or is it a kind of scam? (the normal price used to be around 350 HKD).
I know someone who had to show pretty much all the passports they'd ever been issued with. The rules allow for photocopies of just the relevant pages of the relevant documents. At least under some circumstances (may not apply in HK)
According to china-embassy.org
If you have obtained Chinese visas before and want to apply for a Chinese visa with a renewed foreign passport that does not contain any Chinese visa, you should present the photocopy of the previous passport's data page and the photo page if it is separate, as well as the previous Chinese visa page. (If your name on the current passport differs from that on the previous one, you must provide an official document of name change.)
Whether the fees you were quoted to work around your lack of documents are expected I don't know. Fees seem to depend on nationality of applicant as well as on other factors.
I don't know about the current situation in HK, last time I looked the agents wanted far more than it would cost in my home country for a visa, going to the official processing centre, especially for more entries and longer time (I think 3x or 5x as much, so not far out of line with your quote).
I do know that the embassy uses previous entries as one of the things they use to decide how long to give and how many entries, so if you are requesting a long term multiple-entry (or even double entry visa, depending on your nationality) it would likely make a difference.
My advice is to shop around a bit, there are various agents in HK, some of them reputable. If you are getting similar numbers from others (or they say they can't do it), that's the market price. CTS (China Travel Service) is an official state run company, so a scam is quite unlikely, however they might well not be the cheapest.
then they asked me for 3500 HKD
That's not a scam. That's how it works with Chinese visas, at least in Hong Kong.
You can get a visa if you meet all the requirements. If you don't meet the requirements, you can still get the visa via the underground methods for a much larger fee (the agency will use their connections within the embassy).
3500 seems reasonable. For 5000-6000 you can get a one-year no-questions-asked visa.
I had to show the previous passport too. It seems reasonable, since some people changed their name and got a new passport to circumvent an entry ban.