Firstly, do they know that if you sat there doing nothing you will save them money? I wouldn't divulge this to them if they're unaware.

If you have a specialist skill, you're in demand. You can use this as leverage. You'll know straight away how the probation meeting is going, if it's not going so well (which I doubt as you've stated you're saving them money) but they wish to keep you on, I suggest you request another review in x amount of time and provided you improve and meet or exceed their expectations you wish to have your salary reviewed to match market rates and what's expected of you going forward.

If the meeting goes well, I would confidently request that given you're happy to continue working there and they're obviously pleased with your performance. Make a point of highlighting the savings you have created for them, and inform them you'll continue to find new and innovate ways for them to save money, therefore you would like your salary matched to the going market rate to reflect your capabilities.

Go in on the higher end, expect push-back, be open to negotiation, be friendly yet firm and confident. You'll likely receive the standard response that the economy isn't doing very well, so we all need to buckle up for what's ahead. But honestly, you can counter that with saying that's all the more reason they're going to need to you and you wish to be paid adequately for your skills.

Experience: HR/recruitment/manager