Highly Tailored, crop specific fertilisers will tend to have better results, and are obviously much more efficient, since one has absolute control on the "nutritional" growth factors if you will.
I know that words like "man-made", "artificial", "synthetic","chemical" tend to carry a negative connotation around, due to the fact that they are usually synonym with pesticides, or because they are linked with the GMO's industry and large corporations which are known to choose profits over ethics. However, my point here is not to discuss the ethical problems with the agriculture industry, but only to assert that the purist trend within 'organic veganism' is damaging to the cause, and science/tech based mechanisms do yield better results while simultaneously maintaining product quality.
Here's a PDF link of a study on MDPI. (warning, it's a direct download)
We actually found that Organic+Inorganic mix seems to be the best treatment (Greater Yield ^ Less Dosage). Notice however, that this was a combination of Vermicompost+Inorganic, which from my POV will still be vegan since the usage of non-conscious beings for nutrition seems to be in accord with the definition of veganism.
There are however several factors which this study doesn't seem to take into account(or at least is not reported), like price, ecological footprint in production.
One might argue that all things considered, vermicompost will always be cheaper and have a lesser impact on the environment since it can be easily attained by farmers and even produced within the farm itself. Whereas for chemical fertilisers, farmers have to rely on the prices set by the chemical industry conglomerates, which also have a higher environmental impact.
With this in mind, I would argue that in a (utopian)non-market driven, or ethical-market driven society the answer to:
It is sometimes claimed that materials such as manure are important to attain high yields of crops.
Not necessarily true, there are other alternatives like vermicomposting which are able to produce organic fertilisers without the usage of animal derivatives. (Note that some large industrial scale companies do use manure, and meat but this is not a requirement. Actually large quantities of dead animals are not incentivised)
without having to spend significantly more money on vegan farming?
This depends on the market. It is like the fossil fuels-renewables debate. Renewables are still expensive because there is a lack of investment, and the interests of the fossil fuels industry have much greater political influence.
Here's a great article by National Geographic, with Germany as a study subject.
If farmers stopped using inorganic+pesticides and demanded more mixed-pesticide free alternatives, this would cause an industry shift. More demand, lower prices.