I'd say $50/hr is a bit on the low end. I know larger machines in my area bring in over $100 with drive time one-way considered working hours. Or instead of drive time, a minimum of X amount of hours or X amount for hauling charges. My guess is you would burn at least 2 gallons of fuel or more per hour. Bringing fuel charges to at least $10/hr, operator time at least $20 since you know what your doing. Wear and tear on the truck/trailer to haul the machine around, plus profit should bring you close to the $65-$75 mark. Call around to some local excavators to get current prices. From there you can get a idea how to charge since they have to pay a employee, and all related expenses including workmans comp, unemp taxes etc. Which my guess would be around $30 or more per hr. And of course you will be much cheaper since it sounds like your just out to make some extra money. As far as attachments go, it sounds like you know your way around a bobcat. A bucket is all thats really needed for an experienced operator, unless of course your doing something really out of the ordinary. And renting attachments is always a option for oddities. Good luck.
I agree with most of reaperman's comments.
What you need to charge is based on your costs, risk and desired profit.
Costs: you need to acknowledge that everything wears out. You must consider not only depreciation of all the equipment you use but the costs of maintenance and replacement of normal wear items. This includes the truck, trailer, loader, attachments and everything you use on the job. If you don't account for all costs, you will end up putting some money in your pocket but in reality lose money on the job.
Risk: I would advise you to research your states laws pertaining to businesses and liability. In my state if you do not establish a legal business separate from yourself you are risking everything you own if something goes wrong. Limited Liability Corporations (LLC's) are relatively easy to establish. The business becomes a separate legal entity from yourself. If there is a problem and you get sued, the business assets are at risk; not your personal and real property (like your house and savings).
Even with an LLC, insurance should be considered mandatory. Be careful when you establish your business's scope of work. If you include excavation (as defined by your state, is accessing soil a certain distance below surface grade) your insurance costs can be significantly higher. If you limit your scope and jobs above that depth you may be able to establish your business as a Landscaper. Insurance costs are less since the risk is less. Insurance companies are very aware of the costs of damaging any type of underground utilities. Damage to fiber optic cables can cost a company tens of thousands of dollars per hour while service is interrupted. If you not operating as an insured business entity, you will be liable for these costs personally.
Also check on the One Call Laws in your state. Even if the property owner calls for a locate where the work will be accomplished, some states require the business that will actually perform the work to have their own locate ticket on record. There can be significant fines to you if you don't have an open ticket.
Finally
Profit: once you can put the numbers to your costs including:
1. Equipment Operation, Depreciation and Maintenance Costs.
2. Administrative Costs of running the business (licenses, insurance, administrative, taxes, billing, advertising, etc.)
3. Cost of getting you and your equipment to and from the job.
4. Risk Contingency Costs
5. What you are charging for your Labor
You can now add what you desire for profit. This is not your labor cost, but what your business gains after all costs. For a business to be viable you must be able to be profitable. You can't base your rates on what other businesses are charging. There will always be someone who will do the work cheaper. Remember you are selling a service. How you market and provide that service allows successful businesses to charge more than the cheaper guys and still stay busy making a profit.
If you are curious about how equipment operating costs are calculated, the Army Corps of Engineers has a large pamphlet
Construction Equipment Ownership and Operating Expense Schedule. The US is divided into 12 regions. It explains how to calculate your equipment costs specific to your region of the US and includes worksheets you can use to calculate the costs.
A good friend of mine had a one man "Landscaping" business. (Pickup, dump truck, trailer, Bobcat and Cat 920 loader). His costs to just be in business were $5000 per year. He needed to make $5000 beyond all expenses before he was even profitable for the year. Up to that point it appears you are making money pocketing your hourly wage, but the business is in the red.
Ask yourself what are your goals and how much you are willing to risk to achieve them. We have many guys in our area operating without licenses or insurance and low bidding jobs. The interesting fact is they don't last long. They are continuously replaced by others trying the same thing...