One of the best known ancient Roman spiced sausage recipes is Lucanian sausage, from Apicius:

Lucanian sausage or meat pudding are made similar to the above: crush pepper, cumin, savory, rue, parsley, condiment [mixed herbs?], laurel berries and broth [fish sauce (liquamen)]; mix with finely chopped fresh Pork and pound well with broth [fish sauce]. To this mixture, being rich, add whole pepper and nuts. When filling casings carefully p70 push the meat through. Hang sausage up to smoke.

http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Apicius/2*.html

For more on this recipe, see https://siglindesarts.wordpress.com/2014/12/22/lucanian-sausage-apecius-versions/ which quotes a modernised version giving quantities. This uses 1/2tsp of pepper (about 1g) for 1lb of meat. The only significantly expensive spice in the recipe is the pepper (which is imported); the others being locally produced.

Pepper prices are given in some Roman sources. Pliny the Elder gives 4 denarii per pound (Roman pound, abut 330g) for black pepper, with other varieties being more expensive. This was about 1 week of wages for a soldier. The Price Edict of Diocletian also gives a price: 800 denarii (worth rather less than the denarii of Pliny), about a month's wages for an unskilled labourer, or 2 weeks wages for a skilled labourer. Thus, 1lb of pepper was about 1-4 weeks wages. The 1g of pepper would have been worth about 15 minutes of labour.

The Price Edict gives more useful detail. For the ingredients in the modernised recipe cited above, we have approximately:

• Pork, 1lb: 16 denarii (12 denarii for 1 Roman pound)

• Pepper, 1g: 3 denarii

for the two most expensive ingredients. A spiced sausage should not be much more expensive than a plain sausage. The Edict also gives prices for sausages, plain and spiced:

• Beef sausage: 10 denarii per Roman lb

• Smoked Lucanian beef sausage: 10 denarii per Roman lb

• Smoked Lucanian pork sausage: 16 denarii per Roman lb

A price is given for pork sausage, but per ounce. At 2 denarii per ounce, this is 24 denarii for a Roman lb, this is more expensive that the spiced sausage, but the higher price is due to the small unit. While the pepper prices suggest that the spiced sausage should cost more, the unspiced and spiced beef sausages are the same price. Perhaps the Diocletian recipe used much pepper, or the plain sausage has fewer fillers.

TLDR: If the characters could afford to eat street-food sausages, they could afford the spiced version. The prices were low enough so that the employed should have been able to afford such things, at least in moderation.