Hello. How can i get from Shenzhen to Huidong? And approximately how much it will cost?

Both at Bao'an Bus Station and Fuyong Bus Station, there are direct buses to Huidong. It costs CNY50-65 per person and takes 1.5-2 hours.

At SZ North Station you can get a high speed train to Huidong for about RMB 36, though prices vary depending on time of day and train type. There is a normal speed train and sleeping seat as well. Trains leave every hour or so, often less. You will need to take a bus or taxi from the station to your final destination.
If you are in a group, (or even if you're not) and plan a little ahead, you can try Dida, for which there is an "App", and they will tell you the total cost when you arrange a car. They will pick you up at your hotel or where you wish, and drop you off at your final destination. The price is probably less than RMB 200, but of course it depends on exactly where you are and where you are going. If you can arrange for a Dida car, the total will probably be less than taking a taxi to the train station, taking the train, and a taxi to your final destination. If you can share the cost with others, the per person cost can be less than the taxi /train/taxi method, but not cheaper than the bus.
I would be careful about taking any taxi a long distance by yourself, especially if you are not familiar with the area.