I'm afraid "expenses" doesn't tell me what I need to know. Everyday repair and maintenance costs such as plumbing or electrical repairs or redecoration would have been deductible from the rental income for income tax purposes in each tax year since 1988 and cannot be added to the cost of the property. Improvement costs which can be added to the cost of the property for CGT purposes would be for such things as an extension, the installation of central heating in a house which didn't previously have it or the addition of a conservatory for example, ie things which would enhance the value of the property.
Assuming that the £100,000 is improvements costs, then the overall cost is £177,000. Your parents are, therefore, sitting on a gain of £823,000, £411,500 each. If they gave you and your brother half the property, they would each make a gain of £205,750. The first £11,100 of that would be tax free leaving them each with a net taxable gain of £194,650.
There are two rates of CGT on property, 18% and 28%. The rate or combination of rates that they would each pay would be dependent on the level of their respective incomes in the tax year they make the gift. In the current tax year, 2016/17, a maximum of £32,000 of their respective gains can be taxed at 18% but that figure (£32,000) will be reduced by £1 for every £1 of income they each have in excess of £11,000. Therefore, the CGT liability for each of them will be somewhere between £51,302 (£32,000 @ 18% + £162,650 @ 28%) and £54,502 (£194,650 @ 28%).
The gift of half the property to you and your brother will be a potentially exempt transfer by each of your parents of £250,000 each for Inheritance Tax purposes. So long as they live for seven years after making their respective gifts, the value of those gifts will fall out of their respective estates for IHT purposes, saving 40% in IHT. As you will see here, after three years, the potential IHT charge would start to taper away.
I hope this helps but let me know if you have any further questions.