I'm assuming it's all dollars. There is no $4.57, it's actually $4,567 (but I guess the actual number doesn't matter if you treat them all the same).
spend_mean, spend_min, and spend_max are probably the amounts you'd spend if you were in the mean, min and max respectively. For example, according to this data, the mean amount you have saved up is ~$667,459 (I calculated this backwards) - so for the first year you'd spend 2321 + 0.002102*667459 = $3,724. Likewise if you saved the max in the dataset you'd spend $5,358 instead (total, not in addition to anything).
And yes, the var spending is in addition to const spending, it's explained a bit lower in the article.
spend_mean, spend_min, and spend_max are probably the amounts you'd spend if you were in the mean, min and max respectively. For example, according to this data, the mean amount you have saved up is ~$667,459 (I calculated this backwards) - so for the first year you'd spend 2321 + 0.002102*667459 = $3,724. Likewise if you saved the max in the dataset you'd spend $5,358 instead (total, not in addition to anything).
And yes, the var spending is in addition to const spending, it's explained a bit lower in the article.