There are 3 types of floral arrangements that speak to the farmhouse style, in my opinion, and they all seem to fall into one of two categories: minimal and bare or big and bunchy. The first isn't really a flower arrangement at al,l but an arrangement of branches and possibly even branches with berries or small flowering buds. The second would be a loose arrangement of something droopy like long stem tulips or a a big and happy bunch of hydrangea, maybe even ranunculus or peonies. Third would be the ever so farmy bunch of wild flowers, completely unkempt and ungroomed, and all of these kept in an unconventional vase type. Think ceramic vases, milk jugs, mason jars, mint julep cups (see my Valentine's Day Table-Scape for an image of these) but mainly, think outside the box here. The flowers or branches themselves, in a farmhouse inspired arrangement are only half of the picture. Farmhouse arrangements place an equal importance on the beautiful, utilitarian, or rustic nature of the vessel holding the arrangement.
If fresh flowers are something you can afford to purchase and arrange regularly, then you should enjoy the beauty that brings to your life.
If that isn't a realistic option for you (me included), you don't have to go without. There are options friends! One of my favorite options is the ever so wonderful buying of a real plant for keeping inside for as long as it will thrive there and then transfering it outside to my garden. For me this is a relatively short period (2-3 months so really not that short) since I am not successful as a potted gardener (can never get the watering right). Hydrangeas are a perfect choice for this! You can purchase a small plant from your grocery store, nursery, maybe even your hardware store for $20 or less (aim for $15 if possible). You can then place the plastic pot they come in, inside of another vessel, or you can treat it in one of several ways outlined in my DIY Beachgrass Beautiful post. If you choose a potted plant that comes with a paper wrap you like, then you won't need to do much to it for it to look nice with your decor. If you aren't in love with the paper wrap it has, try turning it inside out and placing it back over the pot (rinse first to remove an dirt residue).
If you would like to make your own arrangement stick to larger flowers and less of them, or do a minimal branchy arrangement. These will be your least expensive options. If you can cut from your yard, do so, but clean and dry well before bringing into your home.
I will provide some links below (click on the images) to sites selling well priced flora and containers, some seperate and some completed already. If you like the look of water in a clear vase, WalMart sells a package of resin in their floral department (or aisle) for $5. This is a great way to make your own realistic arrangements for a quarter of the price (or less sometimes).
Keep in mind that if a pre-made arrangement is priced well but isn't quite right for the look you desire you can dismantle it and use its parts. I do this quite a bit with floral arrangement because I find the flowers themselves to be a bit pricey on occasion. Also, places like Michael's regularly place floral items on sale with heavily discounted prices, so check in frequently to take advantage of these sales. The image at the beginning of this post is a weekly ad for my local store marked down by 40%.
The items below are a perfect example, priced between $15 and $24 from Walmart, you might not be able to purchase this many stems and a vessel for less than this. Keep in mind that stems of silk flowers can run between $1 and $8 or
more. Before you buy something to dismantle it, count the stems and
make sure you are getting a good deal.

Priced at $5.99 from Ikea, these individual faux plants or herbs might add just the touch of green you need.
Priced at $2.99 per stem from Ikea, these Ranunculus would look
wonderful in a vessel with no more than 3 stems necessary or even placed individually in bud
vases. A series of small vases or jars, each with one flower stem,
and scattered about would be wonderful.
Also priced at $2.99 these hydrangea from Ikea come in beautiful colors
and would look wonderful, beheaded and in a shallow dish or bowl.

This bundle of birch branches is priced at $13.99 for a bundle of 25 and stands 36-48" in height from SaveonCrafts.
This bundle with red berries is priced at $7.99 for a group of 4-5 branches and stands at 3' in height from Saveon Crafts
Below are a grouping of vases and vessels from Ikea priced between $1.99 and $2.99, you just can't beat those prices (unless you are shopping at your local dollar store of course)!
These are priced a bit more ranging from $4.99 to $19.99, but as you can see some of them fit the farmhouse theme quite well.

Check your grocery store for a package of well priced Mason Jars, they will most likely be in the baking aisle and are used for canning.
TJ Maxx, Ross and other discount stores will also have a nice collection of glass and ceramic decorative containers...and don't forget about your local dollar store, their prices can't be out done, quite literally!