What primarily affected fashionability during the 17th century was not keeping up with trends - trends require some form of mass communication to be frequently reported and changed, except for a small class of regular consumers that could pass trends around via word of mouth - but use of more expensive materials and more ... moreness. There's an excellent image in Seventeenth-Century Women's Dress Patterns (Susan North and Jenny Tiramani) of a detail from the "panel of the tailors", 1662, from the Ulmer Museum, which I unfortunately cannot find online, that shows the difference between ordinary dress and luxury dress. The tailor is well dressed in a suit of a dark grey fabric, with plenty of the white shirt showing through at the bottom of and a slash in the sleeves, and between the coat and breeches; the customer has even fuller (and whiter) shirtsleeves, and copious amounts of orange silk ribbon loops all over his black suit as trimming. A middle-class man spending more on a best suit might emulate this style of trim in a less excessive way, or buy a few whiter/fuller/finer linen shirts; he could also keep a more conservative cut to his clothing, but invest in a finer wool, possibly dyed black, an expensive color to procure. Women could do essentially the same thing - a more expensive color, better wool, some silk ribbon trim (applied flat rather than in loops). Among the working classes, going up a level in terms of wool fineness was the most likely way of having a better outfit. (At the time, wool was the main fiber used for clothing that wasn't directly on the skin, and it was woven and fulled into numerous fabrics - serge, kersey, russet, etc.)

There was also a thriving secondhand market in London, which would allow people to purchase mended/out-of-style clothing that they would never have been able to afford new. Most metalwork may have been removed in order to be sold as gold and silver alone, but silks and velvets could be available.

Of course, for the setting you're asking about the politics of dressing more finely have to be taken into account re: the Civil War.

The price of fashion during this time is mostly made up by the textile chosen. While someone of good wealth might chose a more expensive fabric like a heavy brocaded silk or gilt embroidered linen, the same basic cut of garment could be made in a sturdy wool or plain linen for an incredible difference in price. Silk, cotton, linen, and wool all come in a wide range of prices. What would be consistent, however, is who is making the clothing. Tailoring is one of the oldest and most populous trades in existence and all people are going to them to have their fitted clothing made. It's the poor farmer who can't afford to take time out of their life to make all of the clothing for the family when they could be earning money, as well as having no knowledge of how to make such garments (no patterns or instruction books). It's not the tailors work that is expensive (their motto is literally "I clothe all"). If a working class person is looking to show off, the easiest way is an investment in a nicer fabric or trimmings.

Choices in garment styles can be made to reduce the amount of fabric needed, such as having a short jacket made instead of a long gown with a full skirt. The cut of the jacket made can still be fashionable, changing the movement of a pair of shears doesn't cost more. Re-use of fabric is also extremely common. That petticoat that wore out at the hem could easily be recut into a jacket, childs gown, waistcoat, etc in a manner where the wear won't show. Now they have a new garment for very little money.