To answer the OP's question, consider
AnkaSoftware.com . Very experienced in C++, C# and several trading platforms.
Define astronomical. An unreliable script kiddie looking for some beer/pizza money will charge $100 for a whole job. An experienced professional will charge that per hour.
I've seen this over and over on the internet. People who can't program need someone else to code something for them but don't want to pay anything. As far as "disappearing with...money" I'd be more concerned with someone disappearing with my code without paying.
All that said, you can check sites such as quantcode.com. There are plenty of programmers there with the background you're looking for.
Most people do not know how to hire programmers. Paying by the job is a recipe for failure, mainly because what you
ask for, what you
meant to ask for, and what you get can be 3 different things. And nether the programmer or you are under no obligation to agree to new terms after the fact. You want to pay by the hour. It seems more expensive, but it is actually cheaper in the long run. And you have way more control over how the project progresses. And you get the source code from the first prototype.
Most people don't know how to define what they want in rational or logical terms. Trust is an issue, but when you pay by the hour that is not too important. You can gauge trust on an ongoing basis. The best you can do is keep [what you think are] special parameters secret. And give the programmer dummy parameters that allow him to properly backtest the function.
Paying by the hour does not necessarily mean that you will pay for each hour worked. Similar to using a car mechanic, there will be an estimate for how much a 'standard' job will be.....and then you usually pay that even if the real time took longer. But if it will be more, then the mechanic will inform you of what the new estimate will be, what it will entail, and you can agree to the new terms or not. So with a programmer, it's similar. You might end up with like 100 hours worth of work (when you include all the chats, screenshots, etc needed to iron out bugs) but maybe only 20 hours were billed.
Many of the freelance websites also suck when it comes to arbitration. Back when vWorker was active, they at least had a very fair and transparent arbitration system. But you paid 20% commissions for all the transparency they offered. Lots of scams to be had.....but also legit people out there. good idea to start out with a very small part of what you want and test to see if the person can code it correctly. Then scale from there.