To figure out whether the cost of custom fibre processing is worth the benefit, get the mill's rates and minimum weight requirements. Multiply those. (We'll assume your fleece weighs the same as the minimum.) Add the cost of the fleece. Add the estimated cost of shipping to the mill and back. Now, take the weight of the fibre and subtract half its amount to get the approximate weight that will be left after washing out the grease. Take the sum of the costs and divide by the final weight of the fibre. That's your cost per unit (pound, ounce, or kilogram). Then compare the cost per unit with the going price of commercial roving.
If you want top, not roving, you are looking at a higher rate per pound and a higher minimum amount of fibre. The process is different and there is more waste. You will get less fibre back with top than with roving.
After you have the cost per unit, factor in how much it's worth it to you to get the following qualities: the fleece that you want as opposed to a homogenized blend from who knows where, the thrill of the hunt for a good fleece without the responsibility for processing one, a local fleece, the considerable time and effort saved not processing by hand, lack of complaints from family members who dislike the smell of raw fleece, and the form of the fibre (roving, top) which cannot be precisely duplicated by hand.
I've haven't done the numbers or sent out a fleece myself. I've talked to a number of people who have done it; they found the cost worth the benefit. The only complaints came when a mill processed an order incorrectly or had a long turnaround time.
Custom processing requires large minimum amounts, so the total amount you pay is greater than what you spend buying commercial roving by the pound or ounce in normal project quantities. For example, a hat takes about 3 ounces, a scarf 6 ounces, a sweater 1-2 pounds. A six pound minimum order means you'll be doing a lot of projects with the three pounds or so you get back. On the positive side, your items will match or coordinate well. However, handspinners will tell you that part of the joy of spinning yarn is sampling different fibres. Variety is preferable to tedious, unrelieved uniformity. So it may not be desirable to acquire a lot of one fibre and tie your money up in it.
Dye may help. That is, dye can make a large amount of the same fibre into a lot of different colours for individual small projects or one large patterned project, and thereby provide variety. Some custom mills dye as well as process; a single colour will be most economical since mills have minimum weight requirements for dye batches.
You can reduce the risk of tying up your money in one large batch of fibre by going in on the purchase with a friend.
There is a halfway measure you can take to get artisanal fibre in small quantities: buy ready-made roving and top by the pound or ounce from local shepherds who send their own fleece out for processing. This has the advantage of letting you see and evaluate the processed fibre in its finished state before you buy, something you don't have when you send fleece out to be processed. You find these sorts of products the same way you find raw fleece.
One final note. If you are thinking about sending out a fleece or fleeces for processing in order to get enough fibre to spin for a sweater and you are new to spinning, talk to experienced handspinners. I am trying to work myself up to this point–the spinning, not the processing–and when talking to handspinners, I've heard that it's certainly a challenge to spin a large amount of consistent yarn.