variation in expression accounted for a SNP -- what's a usual percent?
eQTLs (expression quantitative trait loci) are variants that affect the expression of one or more genes.

There have been several 'genome-wide' studies of SNPs that directly affect expression. The actual effect sizes are hard to pin down in most of them, but in the supplementary data for this paper is a list of the SNPs with the largest effects and coefficients of determination.

There are several common SNPs (cis-acting) that account for >50% of the variation in that genes expression in this (European) population cohort. That being said, that vast majority of variants account for much less variation in the expression, but this is to be expected.

I therefore can fully suppose that a disease-linked SNP could affect the expression of a gene to that degree, particularly if it is an oncogene or tumour suppressor.

Reference:


 Wood AR, Hernandez DG, Nalls MA, Yaghootkar H, Gibbs JR, Harries LW, Chong S, Moore M, Weedon MN, Guralnik JM, et al.. 2011. Allelic heterogeneity and more detailed analyses of known loci explain additional phenotypic variation and reveal complex patterns of association. Human molecular genetics 20: 4082–92.