The cost of genetic testing will depend on how many genes one is investigating.
Breast cancer risk is increased if one of these 13 genes has a mutation in them ATM, BARD1, BRCA1, BRCA2, CDH1, CHEK2, NF1, PALB2, PTEN, RAD51C, RAD51D, STK11, TP53 . It is important to clarify one does not have the risk if you have the gene, everyone has them, it is when they are mutated and unable to do their protective work that risk is increased. Every one of these genes produce a protein, that has a tumour suppressor function, helps repair DNA damage or prevent cells to divide too quickly or too much.
This correction is important because it provides an answer to @purpleme12 's question about what to do with the results. The answer being to change lifestyle and adopt a cancer preventative diet and habits, increased screening for early detection, enter a prophylactic oncology program, ... The action of repairing DNA errors, down regulating cellular pathways is never exclusive to one gene, one protein, one enzyme. If one is deficient there is good of intervention at other levels.
I work in genetic engineering and gene expression and methylation is way more complicated than having a mutation. There is quite a lot you can control. Many with any of these mutation do NOT develop cancer and many without them will develop cancer. Diet and cellular microenvironment such as inflammation, have a huge role in cancer development.
Private labs that offer breast panel testing are Natera, Invitae, Quest Diagnostics, Bio-Reference Laboratories, Ambry Genetics and Progenity. Invitae is probably the cheapest and most popular in healthcare centre and research labs and offers testing to private citizens at US$250 per gene and a one-off $125 genetic consultation fee with a genetic counsellor. For most providers, you just have to spit in a tube. Blood is preferred but they offer saliva as an alternative.
@Loopytiles quite affordable compared to what it was just two years ago, and will go even cheaper in the future.
@grey12 If you are very concerned about breast cancer , concerned enough to contemplate prophylactic mastectomy, try to get a referral for a genetic counsellor, if you can't with the NHS, consider getting the same consultation with a genetic lab provider and they will be able to advise you on which gene to test. Many of the cancer inducing gene mutations have additional manifestations so the lab should be able to cross them off with simple questions.
In the unlikely case you are positive for a mutation, there are still actions to consider, six months interval breast MRI and the LIBRE (Lifestyle intervention for BREast cancer) program promoted in several countries link.springer.com/article/10.1007/s10549-021-06400-7 and even drugs beyond tamoxifen such as mTOR inhibitors once a week www.ncbi.nlm.nih.gov/pmc/articles/PMC5061784/
Cancer is extremely complex and its complexity is at the same time your opportunity. It is a cascade event with several steps in which to intervene.