I would guess that the reason for the expense is: Face.

You can look at it at the cost as a sort of investment.  If you invite the right people and put on a good show, you should make your costs back, plus a little more in tightly stuffed red envelopes.

Was your friend Chinese?.  And he did not give you a full story.  Marriage in China could be expensive, but not necessary so.  Evidently, you will need a heart to heart talk with your GF and explain to her how much money you can afford to expend for a wedding, and stick to it.

Going into some of the details of the expenses you mention, for example an engagement ring is not normally used in China, just a wedding band with or without diamonds.  At local jewelry stores, you can get very nice sets here between 1500 to 6,000 Rmb, under $ 1000 US dollars.

Another expense would be what is generally called the dowry but it is really a drowey, since it is from the groom to the bride's family to compensate them for her "lost" since in China a bride leaves her family to joint the groom's family at the wedding ceremony.  Many Chinese take advantage of this with foreigners, and request it even if lady was already married once.  In every day life today, many do not pay a thing, and depending on the economic status of the bride's family, it could run into thousands.  But a normal amount should b within 5?K to 10K, but I do know one case where 25K was asked and paid.  and heard of another one where 60K was asked, do not know if negotiated or paid.  My guess would be how much are you accepted by the bride's family, and if they do really want the wedding or not.

And the wedding banquet is another thing you must pay, but you also will receive many cash gifts in red envelopes to offset the expense, and you may even break even or come ahead.  As a rule of thumb, it will cost you 100 Rmb per guest at the wedding banquet.  Most guests will give you a red envelope with cash inside, and since your father in law is a government official, guests will not be too stingy with the gift. 

in general, a meeting hall at a restaurant is less expensive than one at a major hotel.  You can save money by buying your hard liquor outside rather than paying the restaurant for it, etc.

As I said at the beginning can spend as much as you want, i have been to weddings that cost under 10K, and also to one that was 225K.  The only difference were the number of guests, and number of dishes serviced.  I had a lot of fun at both, even if one was at a more luxurious surroundings.

  Me and the wife got married two years ago, it cost 10 yuan. I thought that was a bit much but we're still happy, so I guess.............yeh, I guess it was worth it.

I don't know what you were using to calculate costs of a 'wedding'. If you mean the actual cost of the ceremony, that is negligible, indeed when we got married (in China) 8 years ago we made a small profit on the wedding. The more guests you invite the more red envelopes you get and so we made a small profit on that.

Now, if you mean the costs of things you need before she will marry you, or her parents want, that is different. For me the 'dowry' was a nominal lucky money sum of 8,000 RMB. As we were going to the UK house and car weren't a requirement but anyway at that time I had two houses in the UK and a car.  I think if your talking 500K to 900K you must be either paying a stupidly high (in my opinion) 'dowry' or are buying a small apartment as part of it.

If her family want you to buy anything for them, as opposed for your future wife and yourself, personally I'd tell them to take a long walk off a short pier.

her family is related to the government and their lifestyle is different and the people they know are expensive and if i want to marry her i have to pay the parents 88000 RMb plus i need to get a big ceremony in a hotel......and buy the "Three Gold" which is a ring a necklace and earrings and all of them must gold with diamonds on them........need to pay for three days before the ceremony for the family and relatives.....so guess what my round up is almost to 900000 Rmb without the house and the car........i guess my dad will buy the house and the car so i didn't calculate that

.....or i might just sel my house in guangzhou to marry her