What exactly is included in that fee?
As for the marriage; it’s quite straight forward to prepare the documents yourself. First, just get in touch with the Amphur where you would like to marry and ask them what documents they will require - from you and your wife.
You will need a document that shows that you are single / divorced (have the freedom to marry) and it needs to be notarised by your embassy. Read here:
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Then you probably need to have it translated into Thai and legalised by the Ministry of Foreign Affairs. Many officed offer that service. I paid 900 Baht per translated page. A stamp from the MFA costs 400 Baht if I remember correctly.
As for the pre-nup: This is the part that I would hire the lawyer for. I think at my wife‘s law firm it starts at 15,000 THB.
Then once you’re married, I‘m sure, as mentioned before, you can update your civil status at the different agencies. Maybe the embassy will also be helpful.
As for the marriage visa/extension of stay; you will have to prepare quite a few documents. But if you get the handout from the immigration office, you can get there step by step. After marriage you can get a 60 days extension fairly easily with more or less just the passport and marriage certificate. In that time period you can prepare documents, funds etc.