Can I deduct a laptop cost used for my rentals? Do I need to split the cost between all my rentals?

There are many levels to answer this question on. You can definitely take a laptop as a business expense.

Generally it is easier to prove its a business expense if you take a portion as a personal expense unless you have another PC or laptop for personal use.

2nd, upon a potential audit the IRS would want to know why you used it for just one property. It would be easier to explain that the laptop is used to take photos of all the properties, manage e-mails with all tenants or keep the books and records of each unit. If you are taking a loss on one property and are trying to load up the expenses for that property that would be a flag,  So I would allocate the laptop to all your rentals. 

And last, given its such a small expense, the IRS would not be keying in on these types of  items. They will be looking more at rental income versus mortgage costs and maintenance costs, etc...    I hope this helps.

It depends, Miscellaneous Expenses that are used exclusively for the rental: 

1. You can deduct expenses required to manage, conserve or maintain your rental property. You can deduct these expenses even if your property is vacant, as long as you're trying to rent it.
2. Improvements to your property must be depreciated once it is made available for rent.

Split between all properties and in Description make sure you put a clear description of your expense method, if applicable. Writing off on one property would could force you to use it exclusively on that property alone.

@mkochins