How can I make DIY wedding centerpieces that are both classy & cheap?

I would go off white to show off the center pieces more since her colors are green and white. If you can't use candles use faux candles. You can buy a 3 pack at Aldi's if there is one by you, less than $15. Get two of those. Set in the middle of a basket. Buy a garland (way less expensive than individual floral picks and wrap around each candle. This will fill some of the area. Secure both ends around candle with wire. Tuck in some eucalyptus pieces around the edges. A bundle of babies breath will add airiness to the bouquet and go with her green and white theme. If you still want more to fill buy one bunch of white asters or mums and cut off individual flowers (again much cheaper than individual floral picks and tuck a couple in each basket. Easy peasy.

You can do a small glass vase or bowl with water, colored sand, or rocks. If you go water, there are small lights that float in the water and add petals. Large magnolia leaves around the base with sprigs of eucalyptus and peonies among the leaves. Kirkland's has something really pretty IMO that you could mimick, either leave off the glass with candles or put the battery operated tea lights in them (Dollar Tree carries them):

https://www.kirklands.com/product/Home-Decor-Pillows/Floral-Trees/Floral-Candle-Centerpieces/Burlap-Dahlia-and-Peony-Centerpiece/pc/2284/c/2297/sc/2317/176161.uts?cid=cse_google&gclid=EAIaIQobChMI1uuA9PTI5AIVBqSzCh29FANNEAQYAiABEgLzz_D_BwE

I like the idea of the glass bowl with water. I added glass beads to the bottom of mine and then added fresh flowers and floating candles. Since you can't have real candles you would use floating battery lights. Here's what mine looked like for my dinner.

I, too, would go with the off-white table cloths. Have you thought of heading over to your Dollar Tree and finding some different height wine glasses? You might consider the glass beads in the bottom with the LED tea lights nestled inside. A champagne color of bead would really look cool! I also like the basket and greenery idea but I'd also add a square napkin in that red color to really make it pop. The basket should be more like a paper plate holder shape so that the stemware will have enough room to stand up. You can always use double sided tape to keep them secure - then cover that with the greenery. Most of all, have fun with this.

I used $1 store plastic divided platters, oasis, grocery store flowers and an IKEA lantern. Each centerpiece was less than$15. You could cut costs further if you use a pillar candle in the center and choose less expensive flowers.

Some nice ideas from other hometalkers

https://www.hometalk.com/search/posts?filter=wedding%20centerpiece

Just because we are approaching fall pumpkin centerpieces could really look nice https://www.pinterest.com/pin/802274121097089672/

Some of the examples seemed rather informal. Inexpensive can mean a lot of different things. A simple formal look is to get simple cleanlined vase. fill the bottom with rocks or glass stones and add 3 white hydrangeas using leaves or getting some seeded eucalytus to over flow the narrow streamlined vase. Make sure you prepare the hydrangea properly ie cut the stems at an angle, smush them, boil the bottom of the stems in water for about 30 seconds and then in a sink or tub of water put the entire hydrangea head and all in water with a capful of bleach and soak for several hours. White hydrangeas bought at wholefoods, costco are reasonable and greens are cheap or super cheap try to use the leaves from the hyrangea themselves - though many of them are too low down and do not do well "out of water" if you try wiring them near the head. Another gorgeous option is to takea fairly tall vase. Put stones on the bottom and get two off white calla lilies. Cut one taller than the other and twist them into the vase. Add some bear grass or monkey grass twisted near the bottom and left free and voila. The two calla lilies reprsent the bride and groom. if you want a keepsake and are okay with fake flowers here is a third option. Purchase as good quality as you can from the dollar store two to three stems of phalenposis orchids from your dollar store per arrangement. Purchase a pot to "plant" these stems, some moss, floral foam and rocks. Glue some floral foam to the bottom of the pot. Fill around it with some stones to help hold it down. Place your two to three stems in the floral foam. Bend the stems of the orchids so that they look nice from all perspecties. Add some more stones on top to hold down the foam. cover with moss. Hopefully you are able to find a few orchid like leaves to insert on the bottom if possible to make it more realistic. This can be done all for under $10 if you are lucky per arrangement; maybe $5 if you are lucky. i have done some though I spent a bit more for more realistic orchids that cost about $14 cdn and look like the fake ones you buy for over $100.

If your crafty and have a glue gun you can do almost anything get a big bowl,thin a wine glass with the wide bottom turn it up side down hot glue it to the center ,then get a plate hot glue it to the fluted center,another glass glue to that plate ,then a smaller plate on top fill all of this with flowers little pumpkins or what ever you with my niece loved it at her formal wedding Good Luck!!

You could get tall clear vases and put silver or gold balls in them and maybe some perls and white feathers sticking out try michaels crafts

The dollar store has tall streight clear vases