what Canada goose says is right, it depends of how big the genome of the organism is. for example in 2014 they found that the largest genome so far belongs to a Locust (Locusta migratoria),a kind of grasshopper that measures between 40-60 cm, another species with big genome is Amoeba dubia a microscopic organism!!! so you don’t need to be big to have a big genome.
I agree with both Canada Goose and Small Red-eyed Damselfly that it depends on the genome rather than the size of the species. However the cost of sequencing is also down to the method of sequencing older sequencing methods tend to be more expensive than newer methods such as next-generation sequencing as technology become more efficient and cheaper.
You can find more information about sequencing methods at https://en.wikipedia.org/wiki/DNA_sequencing
Very good question! Using the techniques offered by the 25 genome project this should not prove an obstacle. PacBio technique can produce sequencing data over 3 million base pairs in length while illumine gives 200 base reads at very high coverage so that over-lapping sequences can be joined. Combining both sets of data together should enable all the component organisms to have the genomes completed. It would require computer biologists (bioinformatics specialists) to enable this data to be fully analysed along with specialists in Lichen biology and molecular biology.
I think that there are parts when answering this question. The first part related to cost is how large is the genome itself. So as Danish Scurvygrass is a hexaploid, so there are 6 copies of each chromosome, and there are 7 chromosomes, giving a total of 46 chromosomes in total. So the genome is quite complex in its organisation.
So this relates to the analysis of the sequenced genome, or how we make sense of the genes if there are upto 6 copies of each gene. This is why we use computational power for the genome analysis. So the final cost is related both to doing the sequencing itself, and crucially doing the resulting analysis to make sense of the genes themselves.