Low-balling used to be a big problem with me, I still do it sometimes but here's some tricks (this also helps with error margin probs):
Always build in (add) 10-20 percent on top of your estimate to give yourself some room to play AND to avoid being a low-baller. So if your estimating 200-250 (as in your case), then quote 225-260 or if you got a LOT of work then quote 240-275

DO remember to take some off the bill if it takes less than u thought, be nice to ppls the reason for this is so you gots room to play but NO u r not really allowed to play, so to speak.
Here's the thing: You can ALWAYS drop a little off the price later but you will find it VERY difficult to add anything to it (man I have a time getting 5 or 10 extra bucks out of them even if the job took twice as long as I thought).
Far as the fert... I don't like spreading other ppl's fert, not really.
But as part of a whole job I sometimes do it, charge $45/hour and bring the spreader with, then once I get ready to do that part, start the stopwatch: 75 cents/minute and charge to the nearest 5.
Estimate-wise, ask how many bags... Usually I can spread 2 bags every 5 minutes but if it's not my stuff then figure it taking twice as long - estimate $5 / bag then if it takes less time take some $$$ off.
I duno, yous say 150x2... are they ordering cubic yards of fert???
For 1/4 acre (~10k sq. ft.) they likely get with Scott's ~25 lb. covering 5k sq. feet each they prolly get 1-2 bags, I'd estimate 10 - 20 dollars should cover you well for THAT amount (that's TWICE the highest I already figured).