Do you have access to firewood, tree prunings, old barnboard etc. If you can get these types of things free you can stretch your decorating dollar.
Will you be using round or long tables? Long tables are perfect for tablescaping- stretching the centerpiece down the length of the table. Use mason jars filled with casual flower arrangements that look like they were just picked from the garden, add votive candles, pieces of vintage china, silver serving pieces,decorative statues etc.
This pic is from bakerella’s wedding. She said they also used silver cream and sugars filled with babies breath.
Burlap runners also give a rustic/vintage look. You can use them atop white linens,
or place them right on the wood table-use plain or colored burlap
adding a sprig of rosemary or lavender adds a nice touch and neither of them wilt easily
you can add doilies which can likely be borrowed or collected from thrift shops
they are easy to make – there are tutorials here on WeddingBee and on other sites
You could also use slices of firewood under candles for inexpensive centerpieces
sunflowers should be ready for your August wedding
they are high impact flowers- just a few makes a large presence
they can do double duty as aisle decor at the ceremony and then move them to the reception for centerpieces
you can make cute placecard holders from branches
if you have the time and inclination you could collect mismatched china- it gives a beautiful look
you could also make your own napkins from pretty floral prints as in the pic above
cafe lights can be strung from the rafters-with or without paper lanterns
or you could hang umbrellas upside down from the celing
These are just a few ideas not knowing much about you or your colors scheme, theme etc.
Let me know if you want any other suggestions.